Revise API for partition bound search functions.

Revise API for partition bound search functions.

Similar to what commit b0229235564fbe3a9b1cc115ea738a07e274bf30 for a
different set of functions, pass the required bits of the PartitionKey
instead of the whole thing.  This allows these functions to be used
without needing the PartitionKey to be available.

Amit Langote.  The larger patch series of which this patch is a part
has been reviewed and tested by Ashutosh Bapat, David Rowley, Dilip
Kumar, Jesper Pedersen, Rajkumar Raghuwanshi, Beena Emerson, Kyotaro
Horiguchi, Álvaro Herrera, and me, but especially and in great detail
by David Rowley.

Discussion: http://postgr.es/m/098b9c71-1915-1a2a-8d52-1a7a50ce79e8@lab.ntt.co.jp
Discussion: http://postgr.es/m/1f6498e8-377f-d077-e791-5dc84dba2c00@lab.ntt.co.jp