Factor out X-GM-MSGID dedupe code.
Factor out X-GM-MSGID dedupe code.

Turns out polling updates weren't doing this after all.

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