agent: update Agent.Heartbeat comment
systemd: remove unused chan field
pkg: add Set iface w/ safe and unsafe impls
unit: Add GetUnitStates func to UnitManager
agent: add UnitStatePublisher
unit: add UnitStateGenerator
agent: switch to UnitStatePublisher/Generator
systemd: drop EventStream
agent: remove unnecessary goroutine
Merge pull request #651 from bcwaldon/agent-unit-events

Move unit event generation into fleet