pgsql: Start Hot Standby faster when initial snapshot is incomplete.

Start Hot Standby faster when initial snapshot is incomplete.
If the initial snapshot had overflowed then we can start whenever
the latest snapshot is empty, not overflowed or as we did already,
start when the xmin on primary was higher than xmax of our starting
snapshot, which proves we have full snapshot data.

Bug report by Chris Redekop

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/93b915b3d1c1a325a6458f9553a37b89d244262b

Modified Files
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src/backend/storage/ipc/procarray.c |   50 +++++++++++++++++++++-------------
1 files changed, 31 insertions(+), 19 deletions(-)