Added event DatabaseStopping to Db class, part of the Starcounter library, allowing developers to subscribe to an event raised when the code host is being shut down
Added internal method RaiseDatabaseStoppingEvent that we expose to the code host bootstrapping library to allow it to raise the DatabaseStopping event when the code host is about to shut down
Complemented the Starcounter.Bootstrap library with a call that makes sure the Db.DatabaseStopping event to be raised when the code host is shutting down.
Merge branch 'issue/1302' into develop. Addresses #1302 and relates to #1761.