New mechanism of events callbacks dereferencing
Related to #185, #371, !415 (closed)
Our current mechanism of keeping reference to callback after unsubscribe_event is a bit ugly: callback release happens only after user called unsubscribe_event one more time minimum one minute after.
I would propose another mechanism: we create a separate thread for each DeviceProxy, which keeps refs 3 sec after unsubscription and then deletes them.
Unfortunately, I cannot create a test to reproducibly create the problem, described in !415 (closed), so I cannot make a reliable test
Edited by Yury Matveyev