test: enable v2 for python p2p depending on global --v2transport flag

This changes the default behavior, individual tests can overwrite this option.
As a result, it is possible to run the entire test suite with
--v2transport, and all connections to the python p2p will then use it.

Also adjust several tests that are already running with --v2transport in the
test runner (although they actually made v1 connection before this change).
This is done in the same commit so that there isn't an
intermediate commit in which the CI fails.

test/functional/p2p_ibd_stalling.py

@@ -80,7 +80,8 @@ class P2PIBDStallingTest(BitcoinTestFramework):
 
         # Need to wait until 1023 blocks are received - the magic total bytes number is a workaround in lack of an rpc
         # returning the number of downloaded (but not connected) blocks.
-        self.wait_until(lambda: self.total_bytes_recv_for_blocks() == 172761)
+        bytes_recv = 172761 if not self.options.v2transport else 169692
+        self.wait_until(lambda: self.total_bytes_recv_for_blocks() == bytes_recv)
 
         self.all_sync_send_with_ping(peers)
         # If there was a peer marked for stalling, it would get disconnected