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/sendheaders.py

@@ -81,9 +81,9 @@ from test_framework.blocktools import create_block, create_coinbase
 
 direct_fetch_response_time = 0.05
 
-class BaseNode(SingleNodeConnCB):
+class BaseNode(NodeConnCB):
     def __init__(self):
-        SingleNodeConnCB.__init__(self)
+        super().__init__()
         self.last_inv = None
         self.last_headers = None
         self.last_block = None