MEMORY: now allows some of the operations to block

This permits implementations built on top of mmap() on Unix.

Signed-off-by: David Scott <dave.scott@citrix.com>
Add a very basic Unix implementation of the MEMORY interface

This is enough to allow the unit tests to pass with Unix pipes and
Unix mmap() for IPC (admittedly still within process)

Signed-off-by: David Scott <dave.scott@citrix.com>