Differentiate between ClosedBlock (can be reopened) and LockedBlock (cannot).

`ClosedBlock`s still keep the pre-finalised state (i.e. state after the last transaction).
`LockedBlock`s do not. New mining algo needs to reopen these `ClosedBlock`s, however enactment
system does not (and `ClosedBlock`s are slower & more hungry), hence the distinction.