state/apiserver: Split Machiner into subpackage

Minor refactoring to put server-side Machiner
facade into its own subpackage as suggested
previously.
As a side-effect errors and interfaces used
had to be moved into another "common" subpackage
to resolve circular imports.

R=fwereade, jameinel, rog
CC=
https://codereview.appspot.com/10044043