pyln: Use a fair FS lock to throttle node startups

We were getting a couple of starvations, so we need a fair filelock. I
also wasn't too happy with the lock as is, so I hand-coded it quickly.

Should be correct, but the overall timeout will tell us how well we
are doing on CI.

contrib/pyln-testing/pyln/testing/fixtures.py

@@ -198,8 +198,8 @@ def teardown_checks(request):
 
 
 @pytest.fixture
-def throttler():
-    yield Throttler()
+def throttler(test_base_dir):
+    yield Throttler(test_base_dir)
 
 
 @pytest.fixture