In this commit we move the tracking of the outstanding intercepted htlcs
to InterceptableSwitch. This is a preparation for making the htlc
interceptor required.
Required interception involves tracking outstanding htlcs across
multiple grpc client sessions. The per-session routerrpc
forwardInterceptor object is therefore no longer the best place for
that.
This commit ensures htlcs currently held by the interceptor
Are not sent again. This prevents potential races in the user code that
may lead to loosing funds.
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.
