pgsql: Add walreceiver API to get remote server version

Add walreceiver API to get remote server version

Add a separate walreceiver API function walrcv_server_version() to get
the version of the remote server, instead of doing it as part of
walrcv_identify_system(). This allows the server version to be
available even for uses that don't call IDENTIFY_SYSTEM, and it seems
cleaner anyway.

This is for an upcoming patch, not currently used.

Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://www.postgresql.org/message-id/20190115071359.GF1433@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/69039fda837d7a9c78e42b9dd5291d454e71f460

Modified Files
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.../replication/libpqwalreceiver/libpqwalreceiver.c | 19 +++++++++++++------
src/backend/replication/logical/worker.c | 4 +---
src/backend/replication/walreceiver.c | 4 +---
src/include/replication/walreceiver.h | 11 +++++++----
4 files changed, 22 insertions(+), 16 deletions(-)