pgsql: Report WAL flush, not insert, position in replication IDENTIFY_S

Report WAL flush, not insert, position in replication IDENTIFY_SYSTEM

When beginning streaming replication, the client usually issues the
IDENTIFY_SYSTEM command, which used to return the current WAL insert
position. That's not suitable for the intended purpose of that field,
however. pg_receivexlog uses it to start replication from the reported
point, but if it hasn't been flushed to disk yet, it will fail. Change
IDENTIFY_SYSTEM to report the flush position instead.

Backpatch to 9.1 and above. 9.0 doesn't report any WAL position.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3bc4c694271bfe28b909f43d45fe5168c18ad5b8

Modified Files
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doc/src/sgml/protocol.sgml | 2 +-
src/backend/replication/walsender.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)