Fix assertion in autovacuum worker

Hi hackers,

PostgreSQL hit the following assertion during error cleanup, after being 
OOM in dsa_allocate0():

void dshash_detach(dshash_table *hash_table) { 
ASSERT_NO_PARTITION_LOCKS_HELD_BY_ME(hash_table);

called from pgstat_shutdown_hook(), called from shmem_exit(), called 
from proc_exit(), called from the exception handler.

The partition locks got previously acquired by

AutoVacWorkerMain() pgstat_report_autovac() 
pgstat_get_entry_ref_locked() pgstat_get_entry_ref() 
dshash_find_or_insert() resize() resize() locks all partitions so the 
hash table can safely be resized. Then it calls dsa_allocate0(). If 
dsa_allocate0() fails to allocate, it errors out. The exception handler 
calls proc_exit() which normally calls LWLockReleaseAll() via 
AbortTransaction() but only if there's an active transaction. However, 
pgstat_report_autovac() runs before a transaction got started and hence 
LWLockReleaseAll() doesn't run before pgstat_shutdown_hook() is called.

See attached patch for an attempt to fix this issue.

-- 
David Geier
(ServiceNow)