[shmemfix3] editorial: folded long lines.
[shmemfix3] introduced open process for segment info to support memmgr restart

reader/writer versions of mapped files are now persisted in a file and will
be used on (re)start of memmer.
some more initial states and related methods were added.
[shmemfix3] added support for 'validate' command in memmgr builder thread.
[shmemfix3] handle failure in reset_memory_segment more gracefully.

this can fail if the data were corrupted, and it's not good to let
the thread die due to the exception.  we now catch this and try to
re-create a new segment, or at least report the error to the main
thread.
[shmemfix3] added SegmentInfo.loaded, which is needed in memmgr restart.

also completed other small TODOs, mainly documentation.
[shmemfix3] update memmgr's segment init so it begins with validation.
[shmemfix3] update auth so it ignores segment update with inuse-only when not.

This can happen in normal initialization, i.e., both the authoritative
server and the memory manager just starts.  This initial reset command
is just for segment reader modules to be synchronized with the memory
manager, and the actual reset will be delayed until the memory manager
completes building the segment data with any un-applied updates.
[shmemfix3] updated memmgr so it can handle validate-completed, too.
[shmemfix3] updated mapped zt segment to allow clearing the segment.

this will be needed if memmgr fails to initialize the segment data on
restart, while reader modules are currently somehow using the segment.
in this case memmgr will send a segment info update to reset the
segment for synchronization, and, as a result, readers will fail to use
the segment until it's fixed.  that's not good, but this should be a rare
erroneous condition and should be acceptable.
[shmemfix3] catch exception from get_cached_zone_writer.

this can happen due to DB locking if DDNS is heavily updating it.
it now also catches an exception from install().  this can happen
if the locking issue happens in the delayed apply of zone diffs.
[shmem3] Merge commit '82ed3e372a88d1056cb3148a7a1c28710fd64784' into shmem3

Fixed Conflicts:
	src/lib/python/bundy/memmgr/tests/builder_tests.py
[shmem3b] Merge commit 'f0d0c22fa23264e0576dbf9592326955a5633ddf' into shmem3b
[shmem3c] Merge commit 'e89181e90e77b031f7ea0219df417b38f7ff6b94' into shmem3c
Merge pull request #33 from bundy-dns/shmem3c

Shmem3c: auth update for memmgr change