Refactor get_partition_for_tuple a bit.

Refactor get_partition_for_tuple a bit.

Pending patches for both default partitioning and hash partitioning
find the current coding pattern to be inconvenient.  Change it so that
we switch on the partitioning method first and then do whatever is
needed.

Amul Sul, reviewed by Jeevan Ladhe, with a few adjustments by me.

Discussion: http://postgr.es/m/CAAJ_b97mTb=dG2pv6+1ougxEVZFVnZJajW+0QHj46mEE7WsoOQ@mail.gmail.com
Discussion: http://postgr.es/m/CAOgcT0M37CAztEinpvjJc18EdHfm23fw0EG9-36Ya=+rEFUqaQ@mail.gmail.com