Ability to close session

Goal

We represent a session with a SQL optimization process with a context, currently we don't have control over session closing, therefore it's impossible to switch a context (the only option is too wait for an idle session closing).

e.g. developer is finished work on a first query group and wants to start working with a second not related query group, he obviously can reset the session and start a new optimization process over the second group but the session will contain two unrelated optimizations.

We need to give an ability to stop session and start a new one to create a context in sessions.

TODO / How to implement

Add stop session command, e.g. stop

Acceptance criteria