[test] Move MY_RELAY to p2p.py

messages.py is for message and primitive data structures. Specifics
about the test framework's p2p implementation should be in p2p.py.

Also rename to P2P_VERSION_RELAY. Also rename msg_version.nRelay to
relay. In Bitcoin Core, this is referred to as fRelay, since it's a
bool, so this field has always been misnamed.

test/functional/p2p_leak.py

@@ -20,6 +20,7 @@ from test_framework.messages import (
 from test_framework.p2p import (
     P2PInterface,
     P2P_SUBVERSION,
+    P2P_VERSION_RELAY,
 )
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
@@ -128,13 +129,14 @@ class P2PLeakTest(BitcoinTestFramework):
         assert_equal(ver.addrFrom.port, 0)
         assert_equal(ver.addrFrom.ip, '0.0.0.0')
         assert_equal(ver.nStartingHeight, 201)
-        assert_equal(ver.nRelay, 1)
+        assert_equal(ver.relay, 1)
 
         self.log.info('Check that old peers are disconnected')
         p2p_old_peer = self.nodes[0].add_p2p_connection(P2PInterface(), send_version=False, wait_for_verack=False)
         old_version_msg = msg_version()
         old_version_msg.nVersion = 31799
         old_version_msg.strSubVer = P2P_SUBVERSION
+        old_version_msg.relay = P2P_VERSION_RELAY
         with self.nodes[0].assert_debug_log(['peer=3 using obsolete version 31799; disconnecting']):
             p2p_old_peer.send_message(old_version_msg)
             p2p_old_peer.wait_for_disconnect()