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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master

Details
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http://git.postgresql.org/pg/commitdiff/10b7c686e52a6d1bb10194ebf9331ef06f044d46

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