Recover existing instances upon agent restart

Minimal recovery of agents upon restart.   Both running and stopped agents are recovered.  Recovered stopped agents are currently marked as state 'created'.

@peterbourgon @stuartnelson3  @elenadeneva92 

Maybe out of the scope of this PR, but: Please add a `_total` suffix to any counter metrics. This is the convention everywhere else, as it indicates that the metric is a cumulative counter. I would also use the plural form of the thing you are counting, like "container_create_failures", "container_recovers" ("container_recoveries"?), since that makes more sense linguistically.