Avoid aborted IMAP connections when fetching lots of flags.
Avoid aborted IMAP connections when fetching lots of flags.

Notably, Outlook.com IMAP appears to terminate connections if you pass long
sequence sets. Work around this.

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