[PATCH] Fix WAIT FOR LSN cleanup on subtransaction abort

While navigating I noticed  WAIT FOR LSN cleanup is incomplete on 

subtransaction abort. An interrupt such as statement_timeout while 

waiting inside a savepoint leaves stale per-backend wait state, 

causing a later WAIT FOR LSN in the same backend to violate 

the wait-heap invariant and crash an assertion-enabled build.

A small reproducer is:

    BEGIN;
    SAVEPOINT s;
    SET statement_timeout = '100ms';
    WAIT FOR LSN '<future-lsn>' WITH (MODE 'primary_flush');
    ROLLBACK TO s;
    SET statement_timeout = 0;
    WAIT FOR LSN '0/0' WITH (MODE 'primary_flush', TIMEOUT '10ms', NO_THROW);
    COMMIT;

where <future-lsn> can be generated with:

    SELECT pg_current_wal_insert_lsn() + 10000000000;

TRAP: failed Assert("!procInfo->inHeap"), File: "xlogwait.c"

The attached patch mirrors the top-level abort cleanup by calling
WaitLSNCleanup() from AbortSubTransaction(), after LWLockReleaseAll().  It
also adds a TAP test to verify that WAIT FOR LSN can be reused in the same
backend after a statement_timeout and ROLLBACK TO SAVEPOINT.

Thoughts?

Regards,
Ayush