Empty search_path in logical replication apply worker and walsender.

This is like CVE-2018-1058 commit
582edc369c.  Today, a malicious user of a
publisher or subscriber database can invoke arbitrary SQL functions
under an identity running replication, often a superuser.  This fix may
cause "does not exist" or "no schema has been selected to create in"
errors in a replication process.  After upgrading, consider watching
server logs for these errors.  Objects accruing schema qualification in
the wake of the earlier commit are unlikely to need further correction.
Back-patch to v10, which introduced logical replication.

Security: CVE-2020-14349

src/backend/replication/logical/worker.c

@@ -2019,6 +2019,12 @@ ApplyWorkerMain(Datum main_arg)
 											  MyLogicalRepWorker->userid,
 											  0);
 
+	/*
+	 * Set always-secure search path, so malicious users can't redirect user
+	 * code (e.g. pg_index.indexprs).
+	 */
+	SetConfigOption("search_path", "", PGC_SUSET, PGC_S_OVERRIDE);
+
 	/* Load the subscription into persistent memory context. */
 	ApplyContext = AllocSetContextCreate(TopMemoryContext,
 										 "ApplyContext",