lightningd: figure out optimal channel *before* forward_htlc hook.

Otherwise what the hook sees is actually a lie, and if it sets it
we might override it.

The side effect is that we add an explicit "forward_to" field, and
allow hooks to override it.  This lets a *hook* control channel
choice explicitly.

Changelod-Added: Plugins: `htlc_accepted_hook` return can specify what channel to forward htlc to.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/htlc_accepted-fwdto.py

@@ -0,0 +1,31 @@
+#!/usr/bin/env python3
+"""A plugin that tells us to forward HTLCs to a specific channel.
+
+"""
+from pyln.client import Plugin
+
+
+plugin = Plugin()
+
+
+@plugin.hook("htlc_accepted")
+def on_htlc_accepted(htlc, onion, plugin, **kwargs):
+    if plugin.fwdto is None:
+        return {"result": "continue"}
+
+    return {"result": "continue", "forward_to": plugin.fwdto}
+
+
+@plugin.method("setfwdto")
+def setfailonion(plugin, fwdto):
+    """Sets the channel_id to forward to when receiving an incoming HTLC.
+    """
+    plugin.fwdto = fwdto
+
+
+@plugin.init()
+def on_init(**kwargs):
+    plugin.fwdto = None
+
+
+plugin.run()