refactor megolm encryption to improve perceived speed

- allow applications to pre-send decryption keys before the message is sent
- establish new olm sessions with a shorter timeout first, and then re-try in
  the background with a longer timeout without blocking message sending

src/base-apis.js

@@ -1776,10 +1776,13 @@ MatrixBaseApis.prototype.downloadKeysForUsers = function(userIds, opts) {
  *
  * @param {string} [key_algorithm = signed_curve25519]  desired key type
  *
+ * @param {number} [timeout] the time (in milliseconds) to wait for keys from remote
+ *     servers
+ *
  * @return {Promise} Resolves: result object. Rejects: with
  *     an error response ({@link module:http-api.MatrixError}).
  */
-MatrixBaseApis.prototype.claimOneTimeKeys = function(devices, key_algorithm) {
+MatrixBaseApis.prototype.claimOneTimeKeys = function(devices, key_algorithm, timeout) {
     const queries = {};
 
     if (key_algorithm === undefined) {
@@ -1794,6 +1797,9 @@ MatrixBaseApis.prototype.claimOneTimeKeys = function(devices, key_algorithm) {
         query[deviceId] = key_algorithm;
     }
     const content = {one_time_keys: queries};
+    if (timeout) {
+        content.timeout = timeout;
+    }
     const path = "/keys/claim";
     return this._http.authedRequest(undefined, "POST", path, undefined, content);
 };