Fix: config.after_timestamp behavior in InMemorySessionService.get_session() and add a test

[allengour]

Fixes #437

• Slice events[i+1:] instead of events[i] to avoid returning the incorrect index
• Change elif config.after_timestamp to independent if so both config filters can be applied
• Add unit test covering cases:

  pytest tests/unittests/sessions/test_session_service.py -k test_get_session_with_config

  

Updated behavior:

[allengour]

Note, for the test, I did not include SessionServiceType.DATABASE in the service_type parameter because there appears to be a separate issue with sqlalchemy in the source code:

[DeanChensj]

Hi @allengour , could you update your branch so that we can merge? Thanks!