Improve session handling #9

Created a session-factory bound to connexion's underlying sqlalchemy engine and instanciated 2 separate sessions - each for api and sync.

Tests for setting/getting active source succeeded, so assuming that there is no need of calling commit() before the query. Since each session is created in the according context / thread, this approach should also be thread-safe but couldnt verify yet.

Need to retest, thus marking this mr to draft.