Enlarge bit-space for MemoryContextMethodID

Enlarge bit-space for MemoryContextMethodID

Reserve 4 bits for MemoryContextMethodID rather than 3.  3 bits did
technically allow a maximum of 8 memory context types, however, we've
opted to reserve some bit patterns which left us with only 4 slots, all
of which were used.

Here we add another bit which frees up 8 slots for future memory context
types.

In passing, adjust the enum names in MemoryContextMethodID to make it
more clear which ones can be used and which ones are reserved.

Author: Matthias van de Meent, David Rowley
Discussion: https://postgr.es/m/CAApHDvqGSpCU95TmM=Bp=6xjL_nLys4zdZOpfNyWBk97Xrdj2w@mail.gmail.com