registry: improve test coverage of unit_state
Merge pull request #702 from jonboulle/reg

registry: improve test coverage of unit_state
pkg: accept initial values in New*Set functions
unit: implement FakeUnitManager.GetUnitStates
registry: fill in more FakeRegistry methods
agent: rename AgentState to AgentCache
agent: use AgentReconciler model to drive Agent
functional: wait 15s for active units

The new agent reconciler slows things down a bit. The functional
tests reliably fail with 10s, and pass with 15s.