When we re-register, our deviceId might change, which makes our sessions
are no longer valid since the recipient will see us as a new device.
Fixes#388
* Session records are now opaque strings, so treat them that way:
- no more cross checking identity key and session records
- Move hasOpenSession to axolotl wrapper
- Remote registration ids must be fetched async'ly via protocol wrapper
* Implement async AxolotlStore using textsecure.storage
* Add some db stores and move prekeys and signed keys to indexeddb
* Add storage tests
* Rename identityKey storage key from libaxolotl25519KeyidentityKey to
simply identityKey, since it's no longer hardcoded in libaxolotl
* Rework registration and key-generation, keeping logic in libtextsecure
and rendering in options.js.
* Remove key_worker since workers are handled at the libaxolotl level
now
