hash: Implement page-at-a-time scan.

hash: Implement page-at-a-time scan.

Commit 09cb5c0e7d6fbc9dee26dc429e4fc0f2a88e5272 added a similar
optimization to btree back in 2006, but nobody bothered to implement
the same thing for hash indexes, probably because they weren't
WAL-logged and had lots of other performance problems as well.  As
with the corresponding btree case, this eliminates the problem of
potentially needing to refind our position within the page, and cuts
down on pin/unpin traffic as well.

Ashutosh Sharma, reviewed by Alexander Korotkov, Jesper Pedersen,
Amit Kapila, and me.  Some final edits to comments and README by
me.

Discussion: http://postgr.es/m/CAE9k0Pm3KTx93K8_5j6VMzG4h5F+SyknxUwXrN-zqSZ9X8ZS3w@mail.gmail.com