channeld: don't commit until we've seen recent incoming msg, ping if required.

Now sending a ping makes sense: it should force the other end to send
a reply, unblocking the commitment process.

Note that rather than waiting for a reply, we're actually spinning on
a 100ms loop in this case.  But it's simple and it works.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_misc.py

@@ -778,7 +778,6 @@ def test_reserve_enforcement(node_factory, executor):
     )
 
 
-@pytest.mark.xfail(strict=True)
 @unittest.skipIf(not DEVELOPER, "needs dev_disconnect")
 def test_htlc_send_timeout(node_factory, bitcoind):
     """Test that we don't commit an HTLC to an unreachable node."""