We make it explicit in the appropriate Block/Transaction structures, not via a
singleton as C# node does. I think this approach has a bit more potential and
allows better packages reuse for different purposes.
Note that the protocol differs a bit from #895 in its notifications format,
to avoid additional server-side processing we're omitting some metadata like:
* block size and confirmations
* transaction fees, confirmations, block hash and timestamp
* application execution doesn't have ScriptHash populated
Some block fields may also differ in encoding compared to `getblock` results
(like nonce field).
I think these differences are unnoticieable for most use cases, so we can
leave them as is, but it can be changed in the future.
It will be important for proper subscription testing and it doesn't hurt even
though technically we've got two http servers listening after this change (one
is a regular Server's http.Server and one is httptest's Server). Reusing
rpc.Server would be nice, but it requires some changes to Start sequence to
start Listener with net.Listen and then communicate back its resulting
Addr. It's not very convenient especially given that no other code needs it,
so doing these changes just for a bit cleaner testing seems like and
overkill.
Update config appropriately. Update Start comment along the way.
