Add ability to migrate status data across backends

If when we request a LocalStatus folder, the folder has to be created,
we look whether the other backend has data, and if it does we migrate
it to the new backend.

The old backend data is left untouched, so that if you change back say
from sqlite to plaintext, the older data is still there.  That should
not lead to data loss, only a slower sync while the status folder gets
updated.

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

offlineimap/folder/LocalStatusSQLite.py

@@ -144,34 +144,6 @@ class LocalStatusSQLiteFolder(BaseFolder):
         self.connection = sqlite.connect(self.filename,
                                          check_same_thread = False)
 
-        if from_ver == 0:
-            # from_ver==0: no db existent: plain text migration?
-            self.__create_db()
-            # below was derived from repository.getfolderfilename() logic
-            plaintextfilename = os.path.join(
-                self.repository.account.getaccountmeta(),
-                'LocalStatus',
-                self.getfolderbasename())
-            # MIGRATE from plaintext if needed
-            # TODO: adopt for plain-text v2
-            if os.path.exists(plaintextfilename):
-                self.ui._msg('Migrating LocalStatus cache from plain text '
-                             'to sqlite database for %s:%s' %\
-                                 (self.repository, self))
-                file = open(plaintextfilename, "rt")
-                line = file.readline().strip()
-                data = []
-                for line in file.xreadlines():
-                    uid, flags = line.strip().split(':')
-                    uid = long(uid)
-                    flags = ''.join(sorted(flags))
-                    data.append((uid,flags))
-                self.connection.executemany('INSERT INTO status (id,flags) VALUES (?,?)',
-                                       data)
-                self.connection.commit()
-                file.close()
-                os.rename(plaintextfilename, plaintextfilename + ".old")
-
         # Upgrade from database version 1 to version 2
         # This change adds labels and mtime columns, to be used by Gmail IMAP and Maildir folders.
         if from_ver <= 1: