refactor: start breaking out issue handling into their own inbound module (#323)

* Move logout functions into session state

* Move verify_message to a free function

* Move handle_sending_time_accuracy_problem to inbound module

* Move handle_original_sending_time_missing to inbound module

* Move handle_invalid_msg_type to inbound module

* Move handle_incorrect_begin_string to inbound module

* Move handle_incorrect_comp_id to inbound module

* Move handle_sequence_number_too_low to inbound module

* Create test_utils to house helpers for session unit tests

* Add light unit test suite for inbound message processing module

Author: davidsteiner