Add ability for a clean worker shutdown that waits for a session completion.

[mfateev]

Is your feature request related to a problem? Please describe. The clean worker shutdown waits for all currently running activities to complete. But in some cases, there is a need to wait for the whole session, which executes multiple activities to completion.

Describe the solution you'd like Consider adding the capability to wait for an activity session to complete when waiting for the worker.Close()

Describe alternatives you've considered Do not provide such a feature.