pgsql: Restore replication protocol's duplicate command tags

Restore replication protocol's duplicate command tags

I removed the duplicate command tags for START_REPLICATION inadvertently
in commit 07082b08cc5d, but the replication protocol requires them.  The
fact that the replication protocol was broken was not noticed because
all our test cases use an optimized code path that exits early, failing
to verify that the behavior is correct for non-optimized cases.  Put
them back.

Also document this protocol quirk.

Add a test case that shows the failure.  It might still succeed even
without the patch when run on a fast enough server, but it suffices to
show the bug in enough cases that it would be noticed in buildfarm.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reported-by: Henry Hinze <henry.hinze@gmail.com>
Reviewed-by: Petr Jelínek <petr.jelinek@2ndquadrant.com>
Discussion: https://postgr.es/m/16643-eaadeb2a1a58d28c@postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4e9821b6fac5042e872d5397f711a67984b165f8

Modified Files
--------------
doc/src/sgml/protocol.sgml               |  8 +++--
src/backend/replication/logical/worker.c |  1 -
src/backend/replication/walsender.c      |  3 +-
src/test/subscription/t/100_bugs.pl      | 55 +++++++++++++++++++++++++++++++-
4 files changed, 61 insertions(+), 6 deletions(-)