Refactor device key upload

Use another-json instead of awful manual json building. Sign the device keys at
the point of upload, instead of having to keep the signed string in
memory. Only upload device keys once (they are correctly merged with the
one-time keys by synapse).

lib/base-apis.js

@@ -696,6 +696,40 @@ MatrixBaseApis.prototype.search = function(opts, callback) {
     );
 };
 
+// Crypto
+// ======
+
+/**
+ * Upload keys
+ *
+ * @param {Object} content  body of upload request
+ *
+ * @param {Object=} opts
+
+ * @param {string=} opts.device_id  explicit device_id to use for upload
+ *    (default is to use the same as that used during auth).
+ *
+ * @param {module:client.callback=} callback
+ *
+ * @return {module:client.Promise} Resolves: result object. Rejects: with
+ *     an error response ({@link module:http-api.MatrixError}).
+ */
+MatrixBaseApis.prototype.uploadKeysRequest = function(content, opts, callback) {
+    opts = opts || {};
+    var deviceId = opts.device_id;
+    var path;
+    if (deviceId) {
+        path = utils.encodeUri("/keys/upload/$deviceId", {
+            $deviceId: deviceId,
+        });
+    } else {
+        path = "/keys/upload";
+    }
+    return this._http.authedRequestWithPrefix(
+        callback, "POST", path, undefined, content, httpApi.PREFIX_UNSTABLE
+    );
+};
+
 
 // Identity Server Operations
 // ==========================