Remove upload neguid pass from sync logic

In order to optimize performance, we fold the 1st and 2nd pass of our
sync strategy into one. They were essentially doing the same thing:
uploading a message to the other side. The only difference was that in
one case we have a negative UID locally, and in the other case, we have
a positive one already.

This saves some time, as we don't have to run through that function on
IMAP servers anyway (they always have positive UIDs), and 2nd were we
stalling further copying until phase 1 was finished. So uploading a
single new message would prevent us from starting to copy existing
regular messages.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

Changelog.draft.rst

@@ -16,6 +16,10 @@ New Features
 Changes
 -------
 
+* Reduced our sync logic from 4 passes to 3 passes (integrating upload of
+  "new" and "existing" messages into one function). This should result in a
+  slight speedup.
+
 Bug Fixes
 ---------