pgsql: Add function pg_xlog_location_diff to help comparisons

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

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/bc5ac3686580079bd4ea26bf027178786d77a9ee

Modified Files
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doc/src/sgml/func.sgml | 20 +++++++-
src/backend/access/transam/xlogfuncs.c | 90 ++++++++++++++++++++++++++++++++
src/include/access/xlog_internal.h | 1 +
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.h | 3 +
5 files changed, 114 insertions(+), 2 deletions(-)