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.
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
engine: do not resolve offer due to zero bids