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