pgsql: Correct epoch of txid_current() when executed on a Hot Standby s

Correct epoch of txid_current() when executed on a Hot Standby server.
Initialise ckptXidEpoch from starting checkpoint and maintain the correct
value as we roll forwards. This allows GetNextXidAndEpoch() to return the
correct epoch when executed during recovery. Backpatch to 9.0 when the
problem is first observable by a user.

Bug report from Daniel Farina

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/68219aaf6b8c6214e5f4a6124bd5607254aad6b3

Modified Files
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src/backend/access/transam/xlog.c |   30 ++++++++++++++++++++++++++----
1 files changed, 26 insertions(+), 4 deletions(-)