Simplify sync

[Sebastian Spaeth]

We still have the 4-pass synchronization strategy that offlineimap
always had. I believe it was Johannes who pointed out that pass1 is
essentially not needed as it overlaps with pass2 (thanks).

Pass 1 is the upload of "new" messages (with negative UIDS)
Pass 2 is the upload of "existing" messages (positive UIDS)

While pass 1 was ongoing we blocked all threads before we entered pass
2.

Pass 1 is never needed on an IMAP server (all messages always have a
positive UID), it is only when uploading a new message from a Maildir.

I integrated both passes into 1, without loss of functionality. Rather
we should be somewhat faster than before as we do less work in all
cases.

I will send a patch based on current master, but I would suggest we wait
until the next release to actually integrate this.

People that are curious could try it out and review the code already :).

Sebastian