pgsql: Add a few entries to the tail of time mapping, to see old values

Add a few entries to the tail of time mapping, to see old values.

Without a few entries beyond old_snapshot_threshold, the lookup
would often fail, resulting in the more aggressive pruning or
vacuum being skipped often enough to matter. This was very clearly
shown by a python test script posted by Ants Aasma, and was likely
a factor in an earlier but somewhat less clear-cut test case posted
by Jeff Janes.

This patch makes no change to the logic, per se -- it just makes
the array of mapping entries big enough to make lookup misses based
on timing much less likely. An occasional miss is still possible
if a thread stalls for more than 10 minutes, but that does not
create any problem with correctness of behavior. Besides, if
things are so busy that a thread is stalling for more than 10
minutes, it is probably OK to skip the more aggressive cleanup at
that particular point in time.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/7c3e8039f450eb99b3a73272d0a1661195747d1b

Modified Files
--------------
src/backend/utils/time/snapmgr.c | 26 +++++++++++++-------------
src/include/utils/snapmgr.h | 13 +++++++++++++
2 files changed, 26 insertions(+), 13 deletions(-)