Refactor: based: Improve functionization of forward_request()

cib_process_request() is overlong and this helps slightly. The "if
needs_forward" block contained several lines that can be moved inside
forward_request().

While we're here, consolidate the redundant "client not interested"
trace messages into one place, add doxygen for forward_request, and
implement a few other best practices as appropriate.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>