It directly affects node security and the default here MUST BE the safe choice
which is to do the verification. Otherwise it's just dangerous, absent any
VerifyBlocks configuration we'll get an insecure node. This option is not
supposed to be frequently used and it doesn't affect the ability to process
blocks, so breaking compatibility (in a safe manner) should be OK here.
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.
