pgsql: Separate messages for standby replies and hot standby feedback.

Separate messages for standby replies and hot standby feedback.
Allow messages to be sent at different times, and greatly reduce
the frequency of hot standby feedback. Refactor to allow additional
message types.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/06828c5febf3a8f9e94bce5bd84634ce990d299f

Modified Files
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src/backend/replication/walreceiver.c |  104 ++++++++++++++++++++--------
src/backend/replication/walsender.c   |  124 +++++++++++++++++++++------------
src/include/replication/walprotocol.h |   15 ++++-
3 files changed, 168 insertions(+), 75 deletions(-)