Fix: Sometimes, the reconnection logic can hang on a flaky internet connection.

[dshkil]

The root cause is obvious: reconnectingLock was not released when this.isClosed == true. To fix the issue, we can handle the case without using the reconnectingLock.

[CLAassistant]

All committers have signed the CLA.

[davidliu]

Hmm, I don't want to remove the mutex here (there's going to be more mutex use-cases within the RTCEngine). Can you just add unlock statement in the fail-fast cases?