Investigate on Tango Monitor lock taken when push_change_event() is invoked and try to remove this limitation if possible

[bourtemb]

Following discussion from tango-controls/TangoTickets#39, it would be interesting to investigate on why the Tango Monitor lock is taken when push_change_event() is called and to see whether we can find an alternative implementation for push_change_event which would not require to take the Tango Monitor lock.