Clarify sqlalchemy session relationship to db txns.
Clarify sqlalchemy session relationship to db txns.

ErinCall/sync-engine,PriviPK/privipk-sync-engine,ErinCall/sync-engine,EthanBlackburn/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,nylas/sync-engine,PriviPK/privipk-sync-engine,ErinCall/sync-engine,wakermahmud/sync-engine,gale320/sync-engine,rmasters/inbox,rmasters/inbox,PriviPK/privipk-sync-engine,rmasters/inbox,wakermahmud/sync-engine,gale320/sync-engine,PriviPK/privipk-sync-engine,closeio/nylas,closeio/nylas,EthanBlackburn/sync-engine,Eagles2F/sync-engine,closeio/nylas,gale320/sync-engine,gale320/sync-engine,EthanBlackburn/sync-engine,nylas/sync-engine,jobscore/sync-engine,jobscore/sync-engine,EthanBlackburn/sync-engine,wakermahmud/sync-engine,ErinCall/sync-engine,jobscore/sync-engine,ErinCall/sync-engine,rmasters/inbox,Eagles2F/sync-engine,PriviPK/privipk-sync-engine,EthanBlackburn/sync-engine,nylas/sync-engine,wakermahmud/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,jobscore/sync-engine,closeio/nylas,wakermahmud/sync-engine