[fix #28] Send heartbeat message on change stream

To avoid closing change stream connection when no change occurs, start
the connection with asking for heartbeat message. It should be sent over
to the client if no change occurs within one minute.
Add useful extractors for exceptions
Added exists and create methods to Design.
Merge branch 'feature/designUpgrade' of github.com:drdozer/sohva into feature/designUpgrade