Rename some shared memory initialization routines

Rename some shared memory initialization routines

To make them follow the usual naming convention where
FoobarShmemSize() calculates the amount of shared memory needed by
Foobar subsystem, and FoobarShmemInit() performs the initialization.

I didn't rename CreateLWLocks() and InitShmmeIndex(), because they are
a little special. They need to be called before any of the other
ShmemInit() functions, because they set up the shared memory
bookkeeping itself. I also didn't rename InitProcGlobal(), because
unlike other Shmeminit functions, it's not called by individual
backends.

Reviewed-by: Andreas Karlsson
Discussion: https://www.postgresql.org/message-id/c09694ff-2453-47e5-b26c-32a16cd75ce6@iki.fi