Introduce lost state for instances

The `lost` state is meant to be used 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

/cc @bernerdschaefer @typingduck