pgsql: Make logical WAL sender report streaming state appropriately

Make logical WAL sender report streaming state appropriately

WAL senders sending logically-decoded data fail to properly report in
"streaming" state when starting up, hence as long as one extra record is
not replayed, such WAL senders would remain in a "catchup" state, which
is inconsistent with the physical cousin.

This can be easily reproduced by for example using pg_recvlogical and
restarting the upstream server.  The TAP tests have been slightly
modified to detect the failure and strengthened so as future tests also
make sure that a node is in streaming state when waiting for its
catchup.

Backpatch down to 9.4 where this code has been introduced.

Reported-by: Sawada Masahiko
Author: Simon Riggs, Sawada Masahiko
Reviewed-by: Petr Jelinek, Michael Paquier, Vaishnavi Prabakaran
Discussion: https://postgr.es/m/CAD21AoB2ZbCCqOx=bgKMcLrAvs1V0ZMqzs7wBTuDySezTGtMZA@mail.gmail.com

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0414ac6a1eb2e457c8019c5a558bd72b37dede89

Modified Files
--------------
src/backend/replication/walsender.c        | 20 +++++++++++++++-----
src/test/perl/PostgresNode.pm              |  5 +++--
src/test/subscription/t/001_rep_changes.pl |  5 +++++
3 files changed, 23 insertions(+), 7 deletions(-)