registry: simplify conditionless loops
Merge pull request #687 from bcwaldon/clean

registry: simplify conditionless loops
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
registry: dual-publish UnitStates and add associated tests
Merge pull request #638 from jonboulle/638

Scope unit state by machine
registry: use pkg.Set to communicate bids

The JobBid struct is unnecessary. Using a simple string means
we can use pkg.Set to communicate a collection of bids, too.
heart: remove machine state in Heart
agent: send unit events only when necessary
Merge pull request #692 from bcwaldon/unit-state-filtering

agent: send unit events only when necessary
Merge pull request #689 from bcwaldon/bid-set

registry: use pkg.Set to communicate bids
Merge pull request #691 from bcwaldon/heart-purge

heart: remove machine state in Heart