Exim integration

this adds some of the missing endpoints to have exim talking to iAPI, additionally, we add a large set of tests for the socket proxy stuff.

In this process I unified some of the naming and put all mail routing related things under the /mail path. Some endpoints can be removed once this got merged and deployed.

Superseeds/includes !19 (closed) aka the uidcdb endpoint, though usage will be enabled in puppet.

I had to copy some small stuff from !15 (merged) to enable mailman and schleuder AR models, but not more than needed for here. !15 (merged) should be easy to rebase on this. Also once !15 (merged) is merged, some of things can be moved to the resources layer.