Remove SingleNodeConnCB

This commit merges the NodeConnCB and SingleNodeConnCB into a single
class (called NodeConnCB). The original intent for the NodeConnCB was to
be able to have a python 'mininode' connect to multiple running
bitcoinds. This has never been used and can be achieved more easily by
having multiple NodeConns backed by a common datastore if it is ever
needed.

The changes in mininode.py are just code moves (and merging the two
classes into a single class). The code changes in the individual test
cases are changing the subclasses to subclass from NodeConnCB instead of
SingleNodeConnCB. There is a lot of duplicate code in the subclasses
that can be removed in future commits.

test/functional/p2p-mempool.py

@@ -14,7 +14,7 @@ from test_framework.util import *
 
 class TestNode(NodeConnCB):
     def __init__(self):
-        NodeConnCB.__init__(self)
+        super().__init__()
         self.connection = None
         self.ping_counter = 1
         self.last_pong = msg_pong()