Add source and target fn's to CandidateRouteHop

    We add `CandidateRouteHop::source` and
    `CandidateRouteHop::source` functions to point
    to current and next hops in route respectively.
    As we have now `source` and `target`
    available in `CandidateRouteHop` we also
    remove `CandidateRouteHop::id` inputs
    so now they are consumed from `self.target`
    and `self.source` functions.
    In the `add_entry` macro we also remove `source`
    and `target` arguments in favor of `candidate`
    of type `CandidateRouteHop` that holds the
    needed info.

Author: jbesraa