Remove StateDbInterface dependency on go-ethereum-substate

[thaarok]

This change reduce dependency of go-opera-norma on specific modification of go-ethereum, by using custom StateDbInterface type. The change allows to use go-ethereum with much smaller amount of modifications -> should make upgrade of go-ethereum more possible.

• Also bunch of dead code required by the old StateDb interface was removed.
• TxPool tests requires a StateDb instance - using Carmen was creating a dependecy loop -> small local interface + mock of StateDb have been introduced to fix this.
• TestBlockSubscription test removed for now because of hard dependency on old StateDb - needs more complex rewriting of GenerateChain...
• Prometheus metrics not working with carmen removed. (to simplify StateDbInterface)

[HerbertJordan]

Thanks for simplifying the dependencies!

So far I can only judge the style. I am just starting to work myself into the internals of the client. So I rely on you and Jirka to judge the suitability of those changes in the bigger picture.