Merge #17931: test: Fix p2p_invalid_messages failing in Python 3.8 because of warning

f117fb00da747147cddfb071c1427a2754c278cd Replace coroutine with async def in p2p_invalid_messages.py (Elichai Turkel)

Pull request description:

  In Python 3.8 `p2p_invalid_messages.py` fails because of the following warning python produce:
  ```
  2020-01-15T13:02:14.486000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_func_test_3xq0f6uh
  ./test/functional/p2p_invalid_messages.py:154: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
    asyncio.run_coroutine_threadsafe(asyncio.coroutine(swap_magic_bytes)(), NetworkThread.network_event_loop).result()
  2020-01-15T13:02:15.306000Z TestFramework (INFO): Sending a bunch of large, junk messages to test memory exhaustion. May take a bit...
  2020-01-15T13:02:17.971000Z TestFramework (INFO): Waiting for node to drop junk messages.
  2020-01-15T13:02:18.042000Z TestFramework.mininode (WARNING): Connection lost to 127.0.0.1:12826 due to [Errno 104] Connection reset by peer
  2020-01-15T13:02:18.141000Z TestFramework (INFO): Sending a message with incorrect size of 2
  2020-01-15T13:02:18.293000Z TestFramework (INFO): Sending a message with incorrect size of 77
  2020-01-15T13:02:18.344000Z TestFramework.mininode (WARNING): Connection lost to 127.0.0.1:12826 due to [Errno 104] Connection reset by peer
  2020-01-15T13:02:18.445000Z TestFramework (INFO): Sending a message with incorrect size of 78
  2020-01-15T13:02:18.597000Z TestFramework (INFO): Sending a message with incorrect size of 79
  2020-01-15T13:02:18.902000Z TestFramework (INFO): Stopping nodes
  2020-01-15T13:02:19.154000Z TestFramework (INFO): Cleaning up /tmp/bitcoin_func_test_3xq0f6uh on exit
  2020-01-15T13:02:19.154000Z TestFramework (INFO): Tests successful
  ```

  so as it says I replaced the co-routine with `async def` which IIUC is supported since Python 3.5, so this makes the test pass both on 3.5+ and on 3.8
  https://docs.python.org/3.5/library/asyncio-task.html ("The async def type of coroutine was added in Python 3.5, and is recommended if there is no need to support older Python versions")

ACKs for top commit:
  laanwj:
    ACK f117fb00da747147cddfb071c1427a2754c278cd if it passes travis
  fanquake:
    ACK f117fb00da747147cddfb071c1427a2754c278cd - observed the failure (it's the only test that fails) with Python 3.8.1, tested the fix with 3.5.6 and 3.8.1. This is our only usage of `asyncio.coroutine`.

Tree-SHA512: c21d50b23ef4d8a777fd1d9dfe433c85b0b5fff35afbd338817021ffcd42caea64b4c70e46cb3a8a543a1bf2aaa9a6b4f075f6493ab64192bc12bf8bafc54a87

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