With all the other performance work we've been doing, getting a message

into the messagelist is now much, much faster. Intentionally loading the
whole mailbox before we let the user interact with the list is
increasingly painful. A 250 message mailbox takes 2+ seconds to "unlock"
on a modern phone.

Consequently, this commit switches us _back_ to progressive loading of
mailboxes from the synchronous version.

Based on user feedback, we may or may not keep this for the production
release.