fix(base/connection/reducer): clear 'connection' field

Currently the listeners for disconnected and failed connection events
are unsubscribed as soon as the connection is established, so
the CONNECTION_DISCONNECTED is never triggered which would clear the
'connection' field. This commit will clear the 'connection' state on
CONNECTION_WILL_CONNECT. It's needed anyway given that there's no
guarantee on when and if the async disconnect operation will finish.

One issue caused by the 'connection' not cleared was that
CONNECTION_FAILED was not reduced correctly and the reload screen was
not displayed for the following scenario:
1. Join and leave any working conference.
2. Turn off network connectivity on the device.
3. Wait for CONNECTION_FAILED. The reload screen will not be displayed,
   because CONNECTION_FAILED is not reduced correctly, because the old
   'connection' value is still there.

react/features/base/connection/reducer.js

 function _connectionDisconnected(
         state: Object,
         { connection }: { connection: Object }) {
-    if (state.connection !== connection) {
+    const connection_ = _getCurrentConnection(state);
+
+    if (connection_ !== connection) {
         return state;
     }
 
             connection: Object,
             error: ConnectionFailedError
         }) {
-
-    // The current (similar to getCurrentConference in
-    // base/conference/functions.js) connection which is connecting or
-    // connected:
-    const connection_ = state.connection || state.connecting;
+    const connection_ = _getCurrentConnection(state);
 
     if (connection_ && connection_ !== connection) {
         return state;
         { connection }: { connection: Object }) {
     return assign(state, {
         connecting: connection,
+
+        // We don't care if the previous connection has been closed already,
+        // because it's an async process and there's no guarantee if it'll be
+        // done before the new one is established.
+        connection: undefined,
         error: undefined,
         passwordRequired: undefined
     });
     };
 }
 
+/**
+ * The current (similar to getCurrentConference in base/conference/functions.js)
+ * connection which is {@code connection} or {@code connecting}.
+ *
+ * @param {Object} baseConnectionState - The current state of the
+ * {@code 'base/connection'} feature.
+ * @returns {JitsiConnection} - The current {@code JitsiConnection} if any.
+ * @private
+ */
+function _getCurrentConnection(baseConnectionState: Object): ?Object {
+    return baseConnectionState.connection || baseConnectionState.connecting;
+}
+
 /**
  * Reduces a specific redux action {@link SET_LOCATION_URL} of the feature
  * base/connection.