1) Fix bugs that prevented decrypt() from being non-transactional
in some cases.
2) Introduce a new unified storage interface.
3) Transition simultaneous initiate from the "needs refresh"
strategy to one that uses session state resurrection and
promotion.
1) ephemeralKey -> ratchetKey
2) Have the caller specify Alice/Bob orientation.
3) Reorganize verification tag.
4) Remove verification tag from key exchange messages, replace
with signatures in both directions.
1) Use the new /v2/keys API for storing/retrieving prekey bundles.
2) For sessions built with PreKeyBundle and PreKeyWhisperMessage,
use a v3 ratcheting session when available.
