pgsql: Fix memory leak in pgoutput with streamed prepared transactions.

Fix memory leak in pgoutput with streamed prepared transactions.

pgoutput records the toplevel XID in the relation's streamed_txns
list, allocated in CacheMemoryContext, when it sends a relation's
schema during a streamed transaction. We clean up that list at stream
commit and stream abort, but missed stream prepare, so the XIDs stay
around until a relcache invalidation for the relation or until the
walsender exits.

Fix by cleaning up the cache after writing the stream prepare message,
marking the schema as sent as we do at stream commit. The subscriber
applies the spooled relation messages at PREPARE time and its relation
map isn't rolled back by ROLLBACK PREPARED, so we don't need to send
the schema again even if the prepared transaction eventually aborts.

Oversight in commit 63cf61cdeb7b.

Author: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAJTYsWWt5aaKHCnV=jNOwABzspd-WbnhWLndDqtESDQRQ-BkKQ@mail.gmail.com
Backpatch-through: 15

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/274da088e021a92e2ffd2b8938a7b978b2cf11b3

Modified Files
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src/backend/replication/pgoutput/pgoutput.c | 29 ++++++++++++++++-------------
1 file changed, 16 insertions(+), 13 deletions(-)