Make encryption asynchronous

We're going to need to send out a load of messages to distribute the megolm
keys; as a first step, deal with the asynchronicity this will require.

lib/models/event.js

@@ -29,6 +29,10 @@ limitations under the License.
 module.exports.EventStatus = {
     /** The event was not sent and will no longer be retried. */
     NOT_SENT: "not_sent",
+
+    /** The message is being encrypted */
+    ENCRYPTING: "encrypting",
+
     /** The event is in the process of being sent. */
     SENDING: "sending",
     /** The event is in a queue waiting to be sent. */