[COMMITTERS] pgsql: Port single-page btree vacuum logic to hash indexes.

Port single-page btree vacuum logic to hash indexes.

This is advantageous for hash indexes for the same reasons it's good
for btrees: it accelerates space recycling, reducing bloat.

Ashutosh Sharma, reviewed by Amit Kapila and by me.  A bit of
additional hacking by me.

Discussion: http://postgr.es/m/CAE9k0PkRSyzx8dOnokEpUi2A-RFZK72WN0h9DEMv_ut9q6bPRw@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6977b8b7f4dfb40896ff5e2175cad7fdbda862eb

Modified Files
--------------
src/backend/access/hash/README         |   5 +-
src/backend/access/hash/hash.c         |  53 ++++++--
src/backend/access/hash/hash_xlog.c    | 237 +++++++++++++++++++++++++++++++++
src/backend/access/hash/hashinsert.c   | 121 ++++++++++++++++-
src/backend/access/hash/hashsearch.c   |   8 ++
src/backend/access/hash/hashsort.c     |   4 +-
src/backend/access/hash/hashutil.c     |  68 ++++++++++
src/backend/access/rmgrdesc/hashdesc.c |   2 +
src/include/access/hash.h              |  17 ++-
src/include/access/hash_xlog.h         |  19 +++
10 files changed, 517 insertions(+), 17 deletions(-)