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-compactblocks.py

@@ -15,9 +15,9 @@ from test_framework.blocktools import create_block, create_coinbase, add_witness
 from test_framework.script import CScript, OP_TRUE
 
 # TestNode: A peer we use to send messages to bitcoind, and store responses.
-class TestNode(SingleNodeConnCB):
+class TestNode(NodeConnCB):
     def __init__(self):
-        SingleNodeConnCB.__init__(self)
+        super().__init__()
         self.last_sendcmpct = []
         self.last_headers = None
         self.last_inv = None