[shmem7] introduced a new "SYNCHRONIZING2" state for segment info.

for the initial load, it's probably better to make sure that readers
start using after both read and write versions have been fully loaded.
otherwise, it could be possible that loading of the writer version may take
long while new updates are coming.

also, changed the behavior on transitioning to READY; if an outstanding
event exists at that point, immediately start it.  otherwise this event
will be pending until next event happens, which is not good.
[shmem7] set inuse-only for sgmt info update unless loaded() is true.

this is memmgr-side of change for the previous change.  with this change
memmgr tells readers to hold off until the initial load is fully complted
(except for the case of memmgr restart, in which case it'd be okay to let
the existing readers to start using it asap).
[shmem7] corrected typo in docstrings
[shmem7] removed a redundant test assert

it's a leftover from the older version of code and is now meaningless.
Merge pull request #54 from bundy-dns/shmem7

improve robustness of memmgr