Shmem3: extend memmgr so it validates mapped files on startup

specifically, it extends the SegmentInfo class (in lib/python/bundy/memmgr/datasrc_info.py) so it has a few more initial states.  See the updated documentation of the class.  Another purpose of this extension is to introduce a separate "version" file, that remembers the latest files for reader and writer.  With these extensions we'll be able to reuse existing mapped file on restart.

bundy-memmgr was also updated so it'll use these extensions.