Fix folder mapping KeyError when starting some IMAP syncs.
Fix folder mapping KeyError when starting some IMAP syncs.

We were creating the {name: id} mapping for folders before we had
necessarily created folder objects for all folders belonging to an IMAP
account, which meant that sometimes we'd get a KeyError starting up the
sync because we couldn't find a Folder object for a given folder.

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