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
------
master

Details
-------
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(-)