And include some node-specific configurations there with backwards
compatibility. Note that in the future we'll remove Ledger's
fields from the ProtocolConfiguration and it'll be possible to access them in
Blockchain directly (not via .Ledger).
The other option tried was using two configuration types separately, but that
incurs more changes to the codebase, single structure that behaves almost like
the old one is better for backwards compatibility.
Fixes#2676.
This allows to reuse it across different packages.
testchain can't be used because of circular dependencies.
Init() is not changed except for filepath.Join() use instead of direct string
appends which is a better approach anyway. rootpath is required because
current directory will change from package to package.
