IMAP-Resource: Check available UID's first & retrieve 50 time more flags.

This way we avoid the UID fragmentation problem (we try to fetch a lot of
messages that are not available on the server).
It's also a nice first step towards only syncing a certain time-frame.

Retrieving 50 times more flags than full messages should still result in
low memory consumption while greatly reducing roundtrips.

BUG: 334280