In RPC `eth_protocolVersion`, look at the live `EthereumNode` to find which
version of `eth/NN` protocol is active, instead of trusting a compile-time
constant. It's better to check dynamically. GraphQL already does this.
As a result, the RPC code doesn't depend on `eth_protocol` any more.
To make sure there are no more accidental users of the old constant,
`protocolVersion` is no longer exported from `protocol_eth65`.
(The simplest way to support `eth/65` was to make `eth_protocolVersion` use
`protocol_eth65.protocolVersion`, to get 65. But that's silly. More
seriously, when we add another version (`eth/66`) running alongside `eth/65`,
that expression would still compile ok yet return the wrong value, while still
passing the RPC test suite.)
Signed-off-by: Jamie Lokier <jamie@shareable.org>
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