state/apiserver/root.go: cache active facades

state/apiserver/root.go: cache active facades

Since I made Client no longer special, it was now being created again
for every request, which is a bit inefficient. However, since we now
have a centralized location where requests go through, it is pretty easy
to add caching there. And now all Facades are cached. It even comes with
a test that we don't create new objects for the same (Name, version)
couple, but we do create a different one when version changes.

https://code.launchpad.net/~jameinel/juju-core/cache-facade-instances/+merge/220937

Requires: https://code.launchpad.net/~jameinel/juju-core/api-srvRoot-ensures-type/+merge/220615

(do not edit description out of merge proposal)

[jameinel, 2014-05-26 10:15:24]

Please take a look.

[dimitern, 2014-05-26 10:42:59]

LGTM