pgsql: Add WalRcvGetState() to retrieve the state of a WAL receiver

Add WalRcvGetState() to retrieve the state of a WAL receiver

This has come up as useful as an alternative of WalRcvStreaming(), to be
able to do sanity checks based on the state of a WAL receiver.  This
will be used in a follow-up commit.

Author: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/19093-c4fff49a608f82a0@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e0ca61e7c4d55c34d67a3cc6fa0bdea2f41d2cf2

Modified Files
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src/backend/replication/walreceiverfuncs.c | 14 ++++++++++++++
src/include/replication/walreceiver.h      |  1 +
2 files changed, 15 insertions(+)