We introduce a new specific fail resolution error when the
external HTLC interceptor denies the incoming HTLC. Moreover
we introduce a new traffic shaper method which moves the
implementation of asset HTLC to the external layers.
Moreover itests are adopted to reflect this new change.
During testing of this PR I figured out the the htlc interceptor
was not shutdown and started properly in terms of closing the
quit channel but also in terms of the relevant logs.
This commit introduces a new invoice htlc interceptor service
that intercepts invoice HTLCs during their settlement phase. It forwards
HTLCs to a subscribed client to determine their settlement outcomes.
This commit also introduces an interface to facilitate integrating the
interceptor with other packages.
