Synchronously delete drafts in IMAP sync.
Synchronously delete drafts in IMAP sync.

Fixes T2353.

ErinCall/sync-engine,nylas/sync-engine,jobscore/sync-engine,nylas/sync-engine,Eagles2F/sync-engine,closeio/nylas,gale320/sync-engine,nylas/sync-engine,ErinCall/sync-engine,jobscore/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,jobscore/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,gale320/sync-engine,ErinCall/sync-engine,closeio/nylas,jobscore/sync-engine,ErinCall/sync-engine,gale320/sync-engine,gale320/sync-engine,ErinCall/sync-engine,closeio/nylas,closeio/nylas