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.
1) Add plain two-way key exchange support libaxolotl by moving
all the KeyExchangeMessage code there.
2) Move the bulk of KeyExchangeProcessor code to libaxolotl
for setting up sessions based on retrieved prekeys, received
prekeybundles, or exchanged key exchange messages.
1) Break the core cryptography functions out into libaxolotol.
2) The objective for this code is a Java library that isn't
dependent on any Android functions. However, while the
code has been separated from any Android functionality,
it is still an 'android library project' because of the
JNI.
1) Move all the crypto classes from securesms.crypto.
2) Move all the crypto storage from securesms.database.keys
3) Replace the old imported BC code with spongycastle.
