fix(core): add retry logic for the grpc subscription

[vgonkivs]

Closes #4090
Added retry logic for the grpc subscription. In case, the server goes down for some reason, block fetcher will attempt to create a new subscription every 5 seconds.

Screenshot shows that re-subscriptions happens once we receive EOF error

[walldiss]

less code, more tests - love it! Much simpler and easier than it used to be!

[renaynay]

utack - thank you for cleanup!