Gnosis synch monitor fixes and refactor

- [x] Fix the synch monitor in the Gnosis keyper following the updates made in issue [582](https://github.com/shutter-network/rolling-shutter/issues/582)
- [x] Refactor the code to use the same package to avoid duplicating the code across implementations
- [x] Add the check config interval to the gnosis keyper as well (right now it is not being set anywhere)
