Find path and route to the next Trampoline node

In this commit, we expand our forwarding logic to do ad-hoc pathfinding
to subsequent Trampoline nodes, covering both blinded and unblinded scenarios.
We set a constant for retrying payments in Trampoline nodes up to 3 failures.

We further modify our error propagation logic to handle Trampoline forward
HTLC failures.

Co-authored-by: Arik Sosman <git@arik.io>

Author: a-mpch