Periodic service state reports

Currently in kingbird there's no way to say whether one or more of the engines are alive or dead.
This blueprint proposes a mechanism to periodically report engine state and store it in the database.

1. Create a service model containing relevant information about the kingbird engine: id, created_at, updated_at, type, etc.
2. Create a database API for CRUD operations on the service.
3. Implement service reporting and cleanup operations.
4. If kingbird-engine is dead, remove lock for that engine id from the database.