mbnames: add option to write the file once per account

The basic problem is in the context of syncing multiple accounts where
one is fast and the others are slower (due to the number of folders).
When the fast account completes, the other accounts are partially written
through the list and if the file is read during this time, the list can
be useless. However, in the general case, the file is probably left
around from a previous run of offlineimap and is more correct, so add an
option to leave it alone until all syncing is done.

Incremental is still the default since this running offlineimap using
its own timer setup is likely the most common setup. Turning it off
works best with one-shot mode triggered by cron or systemd timers.

Signed-off-by: Ben Boeckel <mathstuf@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/accounts.py

@@ -359,7 +359,7 @@ class SyncableAccount(Account):
                 thr.join()
             # Write out mailbox names if required and not in dry-run mode
             if not self.dryrun:
-                mbnames.write()
+                mbnames.write(False)
             localrepos.forgetfolders()
             remoterepos.forgetfolders()
         except: