Redesign the patch for allocation of shmem space and LWLocks for add-on

Redesign the patch for allocation of shmem space and LWLocks for add-on
modules; the first try was not usable in EXEC_BACKEND builds (e.g.,
Windows).  Instead, just provide some entry points to increase the
allocation requests during postmaster start, and provide a dedicated
LWLock that can be used to synchronize allocation operations performed
by backends.  Per discussion with Marc Munro.