[DIDComm Connector] Code Cleanup

Fix issues from MRs:

• Remove dead code (message.go, peedid.go, coordinateMediation.go, instance.go)
• Use logging (message.go, peerdid.go)
• Use const instead of hard coded strings (mesage.go)
• Remove Hello word controller (router.go)
• Remove not needed brackets (router.go)
• Split callbacks into separate files (callbacks.go)
• Remove not needed files of cassandra
• Adjust empty structs (connection.go)
• Change viper configuration (environmentVariablesGetter.go)
• Change callback return values
• Dockerfile: create smaller image