Display estimated sync time and message count

[rgrove]

Feature request from @akumar via Twitter: "estimate of how long the sync is likely to take (or at least the # of objects synced/not synced)."

This would require pre-scanning all mailboxes being synced in order to gather the necessary info so it would add some time to the sync, but should be doable. Cached mailbox state info can potentially reduce the pre-scanning time.