using the new soledad plugin capablity, mail hooks to the post-sync
event by subscribing to the Meta-Doc type of documents. In this way, we
can create the uid tables and the uid entries needed to keep local
indexes for mail that has been processed in another instance.

however, this won't prevent a conflict if a given mail is received and
processed in two different instances. that is a problem that we still
have to deal with.

Resolves: #6996
Releases: 0.4.0

We cannot use setUpClass when running tests with trial.
But, after all, it's not *so* expensive to initialize a new soledad
instance (since we'll be mostly using the memstore for the tests).

This will need to place a configuration file with:

* userid
* uuid
* password (optional)

Use it for even faster startup times, and running under the native
twisted reactor.