Minimally handle multi-million message mailboxes.
Minimally handle multi-million message mailboxes.

Summary:
It's still really slow to fetch all the g_metadata upfront, but at least we can
prevent it from failing with an IMAP error.

Test Plan: Run IMAP commands on the offending really large Gmail accounts.

Reviewers: spang

Reviewed By: spang

Subscribers: kav-ya

Differential Revision: https://review.inboxapp.com/D1415

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