[WIP] resync-uids script for Gmail.

We've encountered this in the wild several times at this point. It
happens.

As a first step, we provide a script which can be run manually to fix up
a specific folder on a given account. It is more difficult to deal with
a UID resync in the course of a sync, because if it's All Mail which
needs a resync, we need to block the Inbox sync as well while we fix up
the UIDs.