Attempting to implement requests for block data revealed a problem with
the previous connection logic. Block data is requested by sending a
`getdata` message with hashes of the requested blocks; the peer responds
with a sequence of `block` messages with the blocks themselves.
However, this wasn't possible to handle with the previous connection
logic, which could only convert a single Bitcoin message into a
Response. Instead, we factor out the message handling logic into a
Handler, which can statefully accumulate arbitrary data into a Response
and signal completion. This is still pretty ugly but it does work.
As a side effect, the HeartbeatNonceMismatch error is removed; because
the Handler now tries to process messages until it comes to a Response,
it just ignores mismatched nonces (and will eventually time out).
The previous Mempool and Transaction requests were removed but could be
re-added in a different form later. Also, the `Get` prefixes are
removed from `Request` to tidy the name.
Henry de Valence