Prevent long-term memory leakage in autovacuum launcher.

get_database_list() failed to restore the caller's memory context,
instead leaving current context set to TopMemoryContext which is
how CommitTransactionCommand() leaves it.  The callers both think
they are using short-lived contexts, for the express purpose of
not having to worry about cleaning up individual allocations.
The net effect therefore is that supposedly short-lived allocations
could accumulate indefinitely in the launcher's TopMemoryContext.

Although this has been broken for a long time, it seems we didn't
have any obvious memory leak here until v15's rearrangement of the
stats logic.  I (tgl) am not entirely convinced that there's no
other leak at all, though, and we're surely at risk of adding one
in future back-patched fixes.  So back-patch to all supported
branches, even though this may be only a latent bug in pre-v15.

Reid Thompson

Discussion: https://postgr.es/m/972a4e12b68b0f96db514777a150ceef7dcd2e0f.camel@crunchydata.com

src/backend/postmaster/autovacuum.c

@@ -1941,6 +1941,9 @@ get_database_list(void)
 
 	CommitTransactionCommand();
 
+	/* Be sure to restore caller's memory context */
+	MemoryContextSwitchTo(resultcxt);
+
 	return dblist;
 }