Uniter Action(id string) API client/server

This commit adds the ability for Uniter clients to query Actions from
state.State by id.  The response includes a server error message if the
Action is not present on the state server.
Test improvements/tweaks
API server Actions is now a batch query.
Check for unit access before querying database
Addressed concerns in Action API PR 140
Action query now uses tags over the wire
Action API query now properly checks validity of request and exercises
error cases
Now correctly mocks up bad API for apiserver test of bad unit query