lightningd/io_loop_with_timers.c: Move mainloop to its own source file, have chaintopology use it.

Fixes: #2687

tests/test_misc.py

@@ -1401,7 +1401,6 @@ def test_newaddr(node_factory):
     assert both['bech32'].startswith('bcrt1')
 
 
-@pytest.mark.xfail(strict=True)
 def test_bitcoind_fail_first(node_factory, bitcoind, executor):
     """Make sure we handle spurious bitcoin-cli failures during startup