converted all our existing shared memory code to use a tdb database

converted all our existing shared memory code to use a tdb database
instead of either sysv or mmap shared memory or lock files.

this means we can now completely remove
	locking_shm.c
	locking_slow.c
	shmem.c
	shmem_sysv.c
and lots of other things also got simpler

locking.c got a bit larger, but is much better compartmentalised now