deprecate: signed/verified units

The signed/verified units functionality still works, but nasty
warnings are logged to stderr.

Fix #671
pkg: accept initial values in New*Set functions
unit: implement FakeUnitManager.GetUnitStates
registry: fill in more FakeRegistry methods
agent: rename AgentState to AgentCache
agent: unexport UnitStatePublisher.PublishAll
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.
agent: simplify agentCache down to bare minimum
engine: mv dumb-reconciler.go -> dumb_reconciler.go
registry: give Bids a string
offer: ignore partitioned offers
engine: drop unused partitioning code
engine: drop wipReconciler
engine: add clusterState object
engine: use clusterState object
engine: move commands from reconciler to Engine
engine: use task mechanism in reconciliation