Optimizes ConnectorTracker.cleanConnectorMap (#13803)

cleanConnectorMap is a hot method.
This change improves multiple code paths:
1) The global resource handler is no longer get()ed from the session for
   each connector (was in two loops).
2) The ConnectorTracker is no longer get()ed from the UI for each
   connector (was in one loop).
3) values() iterator for ClientConnectors replaces
   keySet() iterator + get(key) call
4) use getUI() from ClientConnector instead manual method.
5) moved removeUnregisteredConnectors to ease inlining and readability.

Change-Id: I25ff5bc32d27d30ecd8d354922ec6e4a02b05921