implVersion property
Version of the implementation of the specification. Nodes are free to ignore this; it
serves only as an indication that the code is different; as long as the other two versions
are the same then while the actual code may be different, it is nonetheless required to
do the same thing.
Non-consensus-breaking optimizations are about the only changes that could be made which
would result in only the impl_version changing.
Implementation
final int implVersion;