pgsql: Empty search_path in logical replication apply worker and walsen

Empty search_path in logical replication apply worker and walsender.

This is like CVE-2018-1058 commit
582edc369cdbd348d68441fc50fa26a84afd0c1a.  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

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/dd5d9951618831aa8f015c6a0ebd3a8f96a9ac32

Modified Files
--------------
.../replication/libpqwalreceiver/libpqwalreceiver.c     | 17 +++++++++++++++++
src/backend/replication/logical/worker.c                |  6 ++++++
src/test/subscription/t/001_rep_changes.pl              |  4 ++++
3 files changed, 27 insertions(+)