We need a session object in order to build a session from a
PreKeyWhisperMessage, so add an empty one when none exists on receiving
a message.
Warning: this will break right now if the session can not be constructed
from the received message.There will be an invalid session which will
break if we try to send using it.
Bundles are now fetched on demand when a session needs to be
established. This should lessen the chance of changes to the bundles
occuring before they're used, as well as lessen the load of fetching
bundles.
Also, the message encryption is now done in a background thread, as this
can be somewhat costly if many sessions are present. This is probably
not going to be an issue in real use, but it's good practice anyway.
