Little cleanup of ShmemInit function names

It's bothered me for a long time that some of the shmem initialization
functions have non-standard names. Most of them are called
FoobarShmemSize() and FoobarShmemInit(), but there are a few exceptions:

InitBufferPool
InitLocks
InitPredicateLocks
CreateSharedProcArray
CreateSharedBackendStatus
CreateSharedInvalidationState

I always have trouble remembering what exactly these functions do and
when get called. But they are the same as all the FoobarShmemInit()
functions, just named differently.

The attached patches rename them to follow the usual naming convention.
The InitLocks() function is refactored a bit more, splitting the
per-backend initialization to a separate InitLockManagerAccess()
function. That's why it's in a separate commit.

--
Heikki Linnakangas
Neon (https://neon.tech)