pgsql: Fix hash index vs "snapshot too old" problemms

Fix hash index vs "snapshot too old" problemms

Hash indexes are not WAL-logged, and so do not maintain the LSN of
index pages.  Since the "snapshot too old" feature counts on
detecting error conditions using the LSN of a table and all indexes
on it, this makes it impossible to safely do early vacuuming on any
table with a hash index, so add this to the tests for whether the
xid used to vacuum a table can be adjusted based on
old_snapshot_threshold.

While at it, add a paragraph to the docs for old_snapshot_threshold
which specifically mentions this and other aspects of the feature
which may otherwise surprise users.

Problem reported and patch reviewed by Amit Kapila

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/2cc41acd8fa3ebb8f0501c6102a253fb7053cf46

Modified Files
--------------
doc/src/sgml/config.sgml             | 13 ++++++++++
src/backend/access/hash/hash.c       |  1 -
src/backend/access/hash/hashsearch.c |  4 ----
src/backend/utils/cache/relcache.c   | 46 ++++++++++++++++++++++++++++++++++++
src/backend/utils/time/snapmgr.c     |  3 ++-
src/include/utils/rel.h              |  1 +
6 files changed, 62 insertions(+), 6 deletions(-)