[shmem4] add param to complete_update to indicate success/failure of update.

and if the update fails in the UPDATING state, move back to READY, instead
of trying to synchronize with readers and COPY the data.  in case diff-based
update fails at the last stage (which invalidates the zone followed by full
reload), it would be better to let the readers keep using the current version
until the next load happens.
[shmem4] return load result to the main memmgr thread

so it can benefit from the previous commit: moving back to READY from
UPDATING when loading fails.
[shmem4] adjusted memmgr so it passes load result to complete_update().

change is trivial, so adjusting the test should be enough.
[shmem4] fix memory leak when ZoneDataLoader::commit() fails.

this situation should be rare, but when it happens the zone data is detached
from the zone table and forgotten.
[shmem4] save 1st 100 diff RRs from journal reader in ZoneDataLoader.

this way we can complete using the journal (possibly with DB transaction)
in the load() phase for typical DDNS cases, reducing the risk of failure
in the commit() phase, which is more expensive.
[shmem4] avoid giant lock in memmgr's build thread loop.

previously it tries to load zone data while holding the lock, and the loading
can be a very time consuing task, so it's not good.
[shmem4] clarify a log message: change xfrout_notify to a debug log.

as it can be too noisy with a busy DDNS server.
Merge pull request #38 from bundy-dns/shmem4

Shmem4: a set of small fixes to shmem implementation