plugins: minimal 'pay' plugin.

I wrote this sync first, then rewrote async, then developed libplugin.
But committing all that just wastes reviewer time, so I present it as
if it was always asnc and using the library helper.

Currently the command it registers is 'pay2', but when it's complete
we'll remove the internal 'pay' and rename it. This does a single
'getroute/sendpay' call.  No retries, no options.

Shockingly, this by itself is almost sufficient to pass our current test
suite with `pay`->`pay2`.

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

tests/test_plugin.py

@@ -108,3 +108,11 @@ def test_failing_plugins():
                 '--plugin={}'.format(p),
                 '--help',
             ])
+
+
+def test_pay_plugin(node_factory):
+    l1, l2 = node_factory.line_graph(2)
+    inv = l2.rpc.invoice(123000, 'label', 'description', 3700)
+
+    res = l1.rpc.pay2(bolt11=inv['bolt11'])
+    assert res['status'] == 'complete'