common/features: clean up feature handling for different cases.

The spec is (RSN!) going to explicitly denote where each feature should
be presented, so create that infrastructure.

Incorporate the new proposed bolt11 features, which need this.

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

tests/test_connection.py

@@ -1835,10 +1835,7 @@ def test_dataloss_protection(node_factory, bitcoind):
 
     l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
     # l1 should send out WIRE_INIT (0010)
-    l1.daemon.wait_for_log(r"\[OUT\] 0010"
-                           # gflen
-                           + format(len(lf) // 2, '04x')
-                           + lf
+    l1.daemon.wait_for_log(r"\[OUT\] 0010.*"
                            # lflen
                            + format(len(lf) // 2, '04x')
                            + lf)