In this commit we add the ability to intercept forwarded htlc packets
straight from the RPC layer. The RPC layer handles a bidrectional stream
that comminucates to the client the intercepted packets and handles its
response by coordinating with the interceptable switch.
As part of the preparation to the switch interceptor feature, this
function is changed to return error instead of error channel that
is closed automatically.
Returning an error channel has become complex to maintain and
implement when adding more asynchronous flows to the switch.
The change doesn't affect the current behavior which logs the
errors as before.
This commit extends the link with a new synchronous delivery point for
local UpdateAddHTLC messages. The switch method SendHTLC is updated to
use this delivery point and thereby becomes a synchronous call.
For MPP payments, synchronous hand-off is important. Otherwise the next
pathfinding round could start without the channel balance updated yet.
