HTTP client should be able to retrieve a unit state via
URL like http://*/state/unitName.
Support GET for stateResource just like unitResource.

Define Get method for UnitState in the schema, and regenerate both
json and go.

Introduce UnitState() for API interface as well as HTTPClient struct.

Protobuf must also its own method GetUnitState().

UnitState() needs to be defined for etcdRegistry, RPCRegistry, and
*RegistryMux*. Also inmemoryRegistry() needs to have UnitState().

Now fleetctl should be able to retrieve a unit state, using
cAPI.UnitState().
Also get rid of getSingleUnitState() to simplify the logic.

We should add new unit tests like TestUnitState() as well as
TestInMemoryUnitState(), to cover the new cases like
EtcdRegistry.UnitState() or inmemoryRegistry.UnitState().

Although checkSystemdActiveState() doesn't have to depend on
cAPI.UnitStates(), it's still impossible to remove additional sleep
in case of error from assertSystemdActiveState(). That's actually a
known issue. Sometimes it simply takes much time until fleetctl became
able to get valid unit states. Adding additional sleeps or tuning the
sleep time wouldn't be a good approach, as an optimal sleep interval
could vary a lot case by case.

To gracefully handle this case, let's do similar checking as done in
functional tests.

* Introduce a new helper waitForState(), just like util.WaitForState()
  from functional tests.
* Squash assertFetchSystemdActiveState() into assertSystemdActiveState()
  to make it retry the assertion periodically up to defaultSleepTime.
* Increase defaultSleepTime from 500 to 2000 msec.
* Remove the additional sleep call as well as cAPI.UnitState() call.

waitForUnitState() has been making use of an additional sleep, in case
assertUnitState() returned error. Now that a helper waitForState() is
available for fleetctl, we don't need to sleep there.

So remove the sleep calls, and instead make assertUnitState() a wrapper
of waitForState() over fetchUnitState(). That way, assertUnitState()
can periodically retry to get a unit, up to defaultSleepTime.
This is also good for consistency in the entire code, as
assertSystemdActiveState() already depends on waitForState().