General client modularization issue

[jochem-brouwer]

The client is currently not modularized very well which makes adding features hard, since it is not a plug-in system. Implementing things like History Purging / importing era1 #3931 will therefore be all-over-the-place. Also keep in mind that we want to multi-thread the client #3695 since it is currently single-threaded. Modularization / splitting up the client in isolated sub-tasks will make this easier or in some cases maybe even doable out-of-the-box 😄 👍

This general issue is supposed to collect all the sub-issues related to client modularization / client cleanup. In general if something is obvious: immediately open a PR to fix this. If this is not the case, open an issue for discussion and link it here as sub-issue.

An example sub issue: logger modularization #3922