Protect list of client plugins from concurrent access

Previously the list of client plugins in the QQmlDebugServer could be
accessed concurrently from receiveMessage() and addService() either
when in non-blocking mode, or if the client uses service discovery or
sends additional hello messages after the first one.

Change-Id: I946243957184210d40ebca728143714c341b1226
Reviewed-by: Kai Koehne <kai.koehne@digia.com>