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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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/efff1cc5fe541ee01488981becd8a54e0f8af49f

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