gossipd: don't send gossip stream, let per-peer daemons read it themselves.

Keeping the uintmap ordering all the broadcastable messages is expensive:
130MB for the million-channels project.  But now we delete obsolete entries
from the store, we can have the per-peer daemons simply read that sequentially
and stream the gossip itself.

This is the most primitive version, where all gossip is streamed;
successive patches will bring back proper handling of timestamp filtering
and initial_routing_sync.

We add a gossip_state field to track what's happening with our gossip
streaming: it's initialized in gossipd, and currently always set, but
once we handle timestamps the per-peer daemon may do it when the first
filter is sent.

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

tests/test_gossip.py

@@ -136,6 +136,8 @@ def test_announce_address(node_factory, bitcoind):
     l1.daemon.wait_for_log(r"\[OUT\] 0101.*004d010102030404d202000000000000000000000000000000002607039216a8b803f3acd758aa260704e00533f3e8f2aedaa8969b3d0fa03a96e857bbb28064dca5e147e934244b9ba50230032607'")
 
 
+# FIXME: Implement timestamp filtering
+@pytest.mark.xfail(strict=True)
 @unittest.skipIf(not DEVELOPER, "needs DEVELOPER=1")
 def test_gossip_timestamp_filter(node_factory, bitcoind):
     # Need full IO logging so we can see gossip (from gossipd and channeld)