pgsql: Add new 'old_snapshot' contrib module.

Add new 'old_snapshot' contrib module.

You can use this to view the contents of the time to XID mapping
which the server maintains when old_snapshot_threshold != -1.
Being able to view that information may be interesting for users,
and it's definitely useful for figuring out whether the mapping
is being maintained correctly. It isn't, so that will need to be
fixed in a subsequent commit.

Patch by me, reviewed by Thomas Munro, Dilip Kumar, Hamid Akhtar.

Discussion: http://postgr.es/m/CA+TgmoY=aqf0zjTD+3dUWYkgMiNDegDLFjo+6ze=Wtpik+3XqA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/aecf5ee2bb36c597d3c6142e367e38d67816c777

Modified Files
--------------
contrib/Makefile | 1 +
contrib/old_snapshot/Makefile | 22 ++++
contrib/old_snapshot/old_snapshot--1.0.sql | 14 +++
contrib/old_snapshot/old_snapshot.control | 5 +
contrib/old_snapshot/time_mapping.c | 159 +++++++++++++++++++++++++++++
doc/src/sgml/contrib.sgml | 1 +
doc/src/sgml/filelist.sgml | 1 +
doc/src/sgml/oldsnapshot.sgml | 33 ++++++
8 files changed, 236 insertions(+)