Introduce lost state for instances

The `lost` state is meant to be used for in cases where external
reasoning about the instance health/state yields errornous results.
Therefore it should be used in scenarios where actual physical state
diverges from our logical representation. It's the only transition
allowed from a client other than the one scheduled the instance
originally.

changes:
* new InsStatus: lost
* interface for transition to lost state
* blocking interface for lost state change
* retrieve list of lost instances
* event emission
Merge pull request #101 from soundcloud/feature/lost-instances

Introduce lost state for instances