BUG #16180: s_lock botteneck when load pg_stat_statements and save=off|on

The following bug has been logged on the website:

Bug reference:      16180
Logged by:          Zhou Digoal
Email address:      digoal@126.com
PostgreSQL version: 12.1
Operating system:   CentOS 7.7 x64
Description:

i use postgresql 12.1 and test load or unload pg_stat_statements. 

when pg_stat_statements.save=off, i found it also call pgss_store

```
Samples: 217K of event 'cpu-clock', 4000 Hz, Event count (approx.):
22185764626 lost: 411555/7098659 drop: 446791/8362859
                                                 
  Children      Self  Shared Object           Symbol

                                        
-   55.76%    55.76%  [kernel]                [k] cpuidle_enter_state

                                        
   - 7.97% 0xffffffff93a000d5

                                        
      - 55.37% 0xffffffff93a59c97

                                        
           0xffffffff93affc2a

                                        
           0xffffffff93a37bce

                                        
           0xffffffff93fbf9ae

                                        
-   29.82%     0.25%  postgres                [.] PostgresMain

                                        
   - 4.16% PostgresMain

                                        
      - 2.00% finish_xact_command

                                        
         - 2.02% CommitTransactionCommand

                                        
            - 2.44% CommitTransaction

                                        
               - 1.90% PreCommit_Portals

                                        
                  - 1.83% PortalDrop

                                        
                     - 1.84% PortalCleanup

                                        
                        - 2.18% pgss_ExecutorEnd

                                        
                           - 10.32% pgss_store

                                        
                              - 10.77% s_lock

                                        
                                   1.30% perform_spin_delay

                                        
      - 1.25% exec_execute_message (inlined)

                                        
-   13.87%     0.03%  postgres                [.] CommitTransactionCommand

                                        
     2.44% CommitTransactionCommand

                                        
      - CommitTransaction

                                        
         - 1.90% PreCommit_Portals

                                        
            - 1.83% PortalDrop

                                        
               - 1.84% PortalCleanup

                                        
                  - 2.18% pgss_ExecutorEnd

                                        
                     - 10.32% pgss_store

                                        
                        - 10.77% s_lock

                                        
                             1.30% perform_spin_delay

                                        
-   13.84%     0.04%  postgres                [.] CommitTransaction

                                        
   - 2.40% CommitTransaction

                                        
      - 1.90% PreCommit_Portals

                                        
         - 1.83% PortalDrop

                                        
            - 1.84% PortalCleanup

                                        
               - 2.18% pgss_ExecutorEnd

                                        
                  - 10.32% pgss_store

                                        
                     - 10.77% s_lock

                                        
                          1.30% perform_spin_delay

                                        
-   13.58%     0.01%  postgres                [.] finish_xact_command

                                        
     1.99% finish_xact_command

                                        
-   12.63%     0.02%  postgres                [.] PreCommit_Portals

                                        
   - 1.87% PreCommit_Portals

                                        
      - 1.83% PortalDrop

                                        
         - 1.84% PortalCleanup   
```

and performance reduce very much.

load pg_stat_statements (104core machine):

```
pgbench -M prepared -n -r -P 1 -c 104 -j 104 -T 120 -S

transaction type: <builtin: select only>
scaling factor: 1000
query mode: prepared
number of clients: 104
number of threads: 104
duration: 120 s
number of transactions actually processed: 67468127
latency average = 0.185 ms
latency stddev = 0.348 ms
tps = 562224.744452 (including connections establishing)
tps = 562300.147713 (excluding connections establishing)
statement latencies in milliseconds:
         0.001  \set aid random(1, 100000 * :scale)
         0.185  SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
```

unload pg_stat_statements:

```
transaction type: <builtin: select only>
scaling factor: 1000
query mode: prepared
number of clients: 104
number of threads: 104
duration: 120 s
number of transactions actually processed: 187563515
latency average = 0.066 ms
latency stddev = 0.014 ms
tps = 1562993.591525 (including connections establishing)
tps = 1563258.811725 (excluding connections establishing)
statement latencies in milliseconds:
         0.001  \set aid random(1, 100000 * :scale)
         0.065  SELECT abalance FROM pgbench_accounts WHERE aid = :aid;
```

why pg 12 load pg_stat_statements and pg_stat_statements.save set to off
will also call pgss_store.