[tests] Make NodeConnCB a subclass of NodeConn

This makes NodeConnCB a subclass of NodeConn, and
removes the need for the client code to know
anything about the implementation details of NodeConnCB.

NodeConn can now be swapped out for any other implementation
of a low-level connection without changing client code.

test/functional/sendheaders.py

@@ -113,6 +113,7 @@ DIRECT_FETCH_RESPONSE_TIME = 0.05
 class BaseNode(NodeConnCB):
     def __init__(self):
         super().__init__()
+
         self.block_announced = False
         self.last_blockhash_announced = None
 
@@ -121,18 +122,18 @@ class BaseNode(NodeConnCB):
         msg = msg_getdata()
         for x in block_hashes:
             msg.inv.append(CInv(2, x))
-        self.connection.send_message(msg)
+        self.send_message(msg)
 
     def send_get_headers(self, locator, hashstop):
         msg = msg_getheaders()
         msg.locator.vHave = locator
         msg.hashstop = hashstop
-        self.connection.send_message(msg)
+        self.send_message(msg)
 
     def send_block_inv(self, blockhash):
         msg = msg_inv()
         msg.inv = [CInv(2, blockhash)]
-        self.connection.send_message(msg)
+        self.send_message(msg)
 
     def send_header_for_blocks(self, new_blocks):
         headers_message = msg_headers()
@@ -155,11 +156,11 @@ class BaseNode(NodeConnCB):
         test_function = lambda: self.last_blockhash_announced == block_hash
         wait_until(test_function, timeout=timeout, lock=mininode_lock)
 
-    def on_inv(self, conn, message):
+    def on_inv(self, message):
         self.block_announced = True
         self.last_blockhash_announced = message.inv[-1].hash
 
-    def on_headers(self, conn, message):
+    def on_headers(self, message):
         if len(message.headers):
             self.block_announced = True
             message.headers[-1].calc_sha256()