gossipd: use htable instead of simple array for node's channels.

For giant nodes, it seems we spend a lot of time memmoving this array.
Normally we'd go for a linked list, but that's actually hard: each
channel has two nodes, so needs two embedded list pointers, and when
iterating there's no good way to figure out which embedded pointer
we'd be using.

So we (ab)use htable; we don't really need an index, but it's good for
cache-friendly iteration (our main operation).  We can actually change
to a hybrid later to avoid the extra allocation for small nodes.

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

gossipd/routing.h

@@ -85,6 +85,26 @@ static inline bool is_halfchan_enabled(const struct half_chan *hc)
 	return is_halfchan_defined(hc) && !(hc->channel_flags & ROUTING_FLAGS_DISABLED);
 }
 
+/* Container for per-node channel pointers.  Better cache performance
+* than uintmap, and we don't need ordering. */
+static inline const struct short_channel_id *chan_map_scid(const struct chan *c)
+{
+	return &c->scid;
+}
+
+static inline size_t hash_scid(const struct short_channel_id *scid)
+{
+	/* scids cost money to generate, so simple hash works here */
+	return (scid->u64 >> 32) ^ (scid->u64 >> 16) ^ scid->u64;
+}
+
+static inline bool chan_eq_scid(const struct chan *c,
+				const struct short_channel_id *scid)
+{
+	return short_channel_id_eq(scid, &c->scid);
+}
+HTABLE_DEFINE_TYPE(struct chan, chan_map_scid, hash_scid, chan_eq_scid, chan_map);
+
 struct node {
 	struct pubkey id;
 
@@ -95,7 +115,7 @@ struct node {
 	struct wireaddr *addresses;
 
 	/* Channels connecting us to other nodes */
-	struct chan **chans;
+	struct chan_map chans;
 
 	/* Temporary data for routefinding. */
 	struct {