Ensuring Goroutine Safety in DTLS Connection Handshake Completion

[jkralik]

This code is not goroutine(thread) safe:
https://github.com/pion/dtls/blob/e20b1620594ce408a20636f455b00ed9a3d69512/conn.go#L1031C3-L1034C4

Because the check c.isHandshakeCompletedSuccessfully is not in the same critical section as c.setHandshakeCompletedSuccessfully(), to fix the issue, c.setHandshakeCompletedSuccessfully() will return a boolean. Internally, it will use CompareAndSwap instead of Store. If it swaps, it returns true and the done channel can be closed.