Instead of passing the PartitionKey, pass just the required bits of
it.  This allows these functions to be used without needing the
PartitionKey to be available, which is important for several
pending patches.

Ashutosh Bapat, reviewed by Amit Langote, with a comment tweak
by me.

Discussion: http://postgr.es/m/3d835ed1-36ab-f06d-0ce8-a76a2bbf7677@lab.ntt.co.jp
Discussion: http://postgr.es/m/b4d88995-094b-320c-b614-2282fae0bf6c@lab.ntt.co.jp