Factor out a function for doing olm encryption

Make a library file with some constants and a function to pack olm-encrypted
events (which we are going to use from megolm)

lib/OlmDevice.js

@@ -294,6 +294,22 @@ OlmDevice.prototype.getSessionIdsForDevice = function(theirDeviceIdentityKey) {
     return utils.keys(sessions);
 };
 
+/**
+ * Get the right olm session id for encrypting messages to the given identity key
+ *
+ * @param {string} theirDeviceIdentityKey Curve25519 identity key for the
+ *     remote device
+ * @return {string?}  session id, or null if no established session
+ */
+OlmDevice.prototype.getSessionIdForDevice = function(theirDeviceIdentityKey) {
+    var sessionIds = this.getSessionIdsForDevice(theirDeviceIdentityKey);
+    if (sessionIds.length === 0) {
+        return null;
+    }
+    // Use the session with the lowest ID.
+    sessionIds.sort();
+    return sessionIds[0];
+};
 
 /**
  * Encrypt an outgoing message using an existing session