Integrate libaxolotl async storage changes

* 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

js/database.js

@@ -33,6 +33,10 @@
                 conversations.createIndex("inbox", "active_at", { unique: false });
                 conversations.createIndex("group", "members", { unique: false, multiEntry: true });
                 conversations.createIndex("type", "type", { unique: false });
+
+                var preKeys = transaction.db.createObjectStore("preKeys");
+                var signedPreKeys = transaction.db.createObjectStore("signedPreKeys");
+
                 next();
             }
         }