Live: clean up root checkpoint injection to mapper

[awfm9]

Currently, we use a Loader to load the root checkpoint. We should get rid of that component, or at least move it to a stand-alone initializer package, like the protocol state initializer. Then, we should simply provide the root trie to the mapper as an optional parameter. When resuming state (see #79), we will not need a root checkpoint, as we can read the root ledger registers from the index database, just like the others.

[Ullaakut]

@awfm9 Could you elaborate? What is the problem, what are potential solutions?

[awfm9]

Doing as part of #79.