Implement change_message_uid

Previously, assigning a new UID to a mapped IMAP or Maildir repository
was done by loading the "local" item, saving it under a new UID and
deleting the old one. This involved lots of disk activity for nothing
more than an effective file rename in Maildirs, and lots of network
usage in the MappedUID cases.

We do this on every upload from a local to a remote item, so that can
potentially be quite expensive. This patch lets backends that support it
(Maildir, MappedUID) efficiently rename the file rather than having to
read the mail content, write it out as a new file and delete the old
file. This speeds up uploads from Maildir and the MappedUID server.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

Changelog.draft.rst

@@ -16,6 +16,10 @@ New Features
 Changes
 -------
 
+* Uploading of Messages from Maildir and IMAP<->IMAP has been made more
+  efficient by renaming files/mapping entries, rather than actually
+  loading and saving the message under a new UID.
+
 Bug Fixes
 ---------