Add function pg_xlog_location_diff to help comparisons

Magnus Hagander <magnus@hagander.net>

Commit: bc5ac3686580079bd4ea26bf027178786d77a9ee
Author: Magnus Hagander <magnus@hagander.net>
Date: 2012-03-04T11:22:38Z
Releases: 9.2.0
Add function pg_xlog_location_diff to help comparisons

Comparing two xlog locations are useful for example when calculating
replication lag.

Euler Taveira de Oliveira, reviewed by Fujii Masao, and some cleanups
from me

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