pgsql: Send new protocol keepalive messages to standby servers.

Send new protocol keepalive messages to standby servers.
Allows streaming replication users to calculate transfer latency
and apply delay via internal functions. No external functions yet.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/64233902d22ba42846397cb7551894217522fad4

Modified Files
--------------
doc/src/sgml/protocol.sgml                 |   48 +++++++++++++++++++++
src/backend/access/transam/xlog.c          |   43 +++++++++++++++++++
src/backend/replication/walreceiver.c      |   47 ++++++++++++++++++++-
src/backend/replication/walreceiverfuncs.c |   63 ++++++++++++++++++++++++++++
src/backend/replication/walsender.c        |   42 ++++++++++++-------
src/include/access/xlog.h                  |    1 +
src/include/replication/walprotocol.h      |   22 ++++++++++
src/include/replication/walreceiver.h      |    8 ++++
8 files changed, 258 insertions(+), 16 deletions(-)