Rename NatsBased + KnownState => ActualState
Rename BulkApiBasedStateProvider and ExpectedStateProvider to DesiredState
Remove the AppStateProvider parent class
Move ActualState@set_desired_state to AppState. Rename ActualState#droplets to ActualState#app_states.
Extract out a droplet registry from Actual State
Rename AppState to Droplet
Refactor droplet analysis
Update varz initialization
Test process heartbeat
Delete droplets from registry that are not in desired state