`_not_acknowledged_results` leaks memory

[guillaumeautran]

Running python3-opcua==0.98.9-1 we are running into a memory leak problem where the _not_acknowledged_results list grows unbounded. The condition occurs when the OPCUA Keep-Alive Interval and the OPCUA Keep-Alive Timeout are set to 500ms.

The problem is that the dictionary declared here: https://github.com/FreeOpcUa/python-opcua/blob/master/opcua/server/internal_subscription.py#L263 has items appended here: https://github.com/FreeOpcUa/python-opcua/blob/master/opcua/server/internal_subscription.py#L334 But the dictionary does not get cleared from here: https://github.com/FreeOpcUa/python-opcua/blob/master/opcua/server/internal_subscription.py#L372 or here : https://github.com/FreeOpcUa/python-opcua/blob/master/opcua/server/internal_subscription.py#L367

Here: https://github.com/FreeOpcUa/python-opcua/blob/583d970bed30b44236418e03b5a6ff6fea481b13/opcua/server/subscription_service.py#L70-L74 is where is should logically be happening (the publication of the ack).

The result is that the dictionary grows unbounded consuming all of the memory of the host.