Previously, libtextsecure would send a sync message automatically
when appropriate. This fails if any recipient has a key conflict
or if our network connection fails mid-send.
Instead, when appropriate, return a the DataMessage encoded as an array
buffer for later syncing. This lets the application choose when to send
it, which we now do after any successful send to a recipient, rather
than after all recipients are successfully sent to.
Eventually we should move the DataMessage protobuf construction and
group sending logic to the application layer entirely, in which case
we wouldn't need libtextsecure to construct the sync message either.
Fixes#408
07702c4ee5