This constant shouldn't be used outside `protocol_eth65`.
When we support multiple `eth/NN` versions side by side, or even just have
multiple code files, there's a risk some code would import just one of the
files (e.g. `protocol_eth65`), use `protocolVersion`, and incorrectly act as
though that version is the one active on the node.
In fact that happened, and now it can't happen. Other code needs to query the
`EtheruemNode` to find what versions are really active.
Signed-off-by: Jamie Lokier <jamie@shareable.org>
936a18b4f4