Merge #11712: [tests] Split NodeConn from NodeConnCB

Merge #11712: [tests] Split NodeConn from NodeConnCB

e9dfa9bcc [tests] Move version message sending from NodeConn to NodeConnCB (John Newbery)
dad596fc3 [tests] Make NodeConnCB a subclass of NodeConn (John Newbery)
e30d40438 [tests] Move only: move NodeConnCB below NodeConn (John Newbery)
4d5059856 [tests] Tidy up mininode (John Newbery)
f2ae6f32a [tests] Remove mininode periodic (half-hour) ping messages (John Newbery)
ec59523c5 [tests] Remove rpc property from TestNode in p2p-segwit.py. (John Newbery)

Pull request description:

  This is the final step in #11518, except for possibly renaming - for motivation, please see that PR.

  If this is merged, then migrating the test framework from asyncore to asyncio should be easier (I say should because I haven't dug too deeply into what would be required).

  Requesting review from @ryanofsky , since he always has good feedback on these refactor PRs, and I'd appreciate his take on this refactor. Note particularly that I've reverted the change suggested here: https://github.com/bitcoin/bitcoin/pull/11182#discussion_r148859555 . The idea, as always, is to present a simple interface to the test writer.

Tree-SHA512: 94dd467a13ec799b101108cf47d4dccb6f6240b601e375e3d785313333bbb389c26072a50759aca663bbf3d6c8b867b99e36ae8800ab8ea115e0496c151926ce