Collapse KeyExchangeMessage and KeyExchangeProcessor interfaces.
Move session construction and KeyExchangeMessage into libaxolotl.

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.