Adapt plain status folder to gmail labels stuff

* Implements Status Folder format v2, with a mechanism to upgrade an
    old statusfolder.

  * Do not warn about Gmail and GmailMaildir needing sqlite backend
    anymore.

  * Clean repository.LocalStatus reusing some code from
    folder.LocalStatus.

  * Change field separator in the plaintext file from ':' to '|'. Now
    the local status stores gmail labels. If they contain field
    separator character (formerly ':'), they get messed up. The new
    character '|' is less likely to appear in a label.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/folder/Gmail.py

@@ -284,7 +284,7 @@ class GmailFolder(IMAPFolder):
                 labels = dstfolder.getmessagelabels(uid)
                 statusfolder.savemessagelabels(uid, labels, mtime=mtime)
 
-            # either statusfolder is not sqlite or dstfolder is not GmailMaildir.
+            # dstfolder is not GmailMaildir.
             except NotImplementedError:
                 return