Make MDClientCtx thread-safe

[frankosterfeld]

MDClientCtx has some threading issues:

The use of the poller thread calling poll() is not thread-safe:

• _pollItems is accessed from the thread without synchronization, that can crash when the MDClient is destroyed while the thread is running. _pollItems is also modified in disconnect()
• Other members such as _subscriptions and _requests are accessed without synchronization.
• I think there should be an explicit poller->request_stop(); poller->join() in ~MDClientCtx

While at it:
if (_subscriptions.contains(receivedEvent.serviceName)) {
_subscriptions.at(receivedEvent.serviceName).callback(receivedEvent); // callback
}
can be replaced by a single lookup (find()) instead of two. Same for _requests below.