Unify execution chain syncing

[jannikluhn]

In Gnosh, we use multiple execution chain state syncing implementations:

• chainsync for keyper sets and eon keys
• validatorsyncer for validator registration messages
• sequencersyncer for encrypted transactions

They contain similar logic, so we should unify them. chainsync is the most general purpose and has the additional feature of detecting reorgs, so we should try to integrate the other two into their framework. To do that, we probably want to make chainsync more abstract and allow "registering" syncers from the outside. Then we only have to make the validatorsyncer and sequencersyncer to implement the required interface.

[ezdac]

I would love to take this one, but with my limited dev-time I'm not sure if this is a fit in terms of delivery time.

By the way, I am currently working on merging the reorg aware chainsync features into the gnosis branch (#444).
In general I think the chainsync package evolved and now is somewhat bloated and hard to understand for what it does, so a refactoring could be beneficial in any case.