Optimize ChangeRecorder to only update 8 bytes on disk when dequeuing.

Rather than saving the whole file (of pending notifications) every
time, just because the first item was dequeued. This reduces disk I/O
considerably, and speeds up processing (especially in the nepomuk
feeder). In the IMAP resource (when marking 2000 emails as read) I
couldn't time a difference since it's mostly network-based anyway.
But at least the disk is usable by other processes meanwhile ;)