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/test_framework/comptool.py

@@ -42,7 +42,7 @@ class RejectResult(object):
 class TestNode(NodeConnCB):
 
     def __init__(self, block_store, tx_store):
-        NodeConnCB.__init__(self)
+        super().__init__()
         self.conn = None
         self.bestblockhash = None
         self.block_store = block_store