If an unfeasible update is applied to an existing, unselected route, we
now skip route selection altogether. Since the route is not selected,
it can obviously not be unselected, and selecting it won't be possible
since its now not feasible.
By immediately returning we also can't notify downstream peers which
have sent a seqno request for this combination. This is fine, since if
there is an outstanding request for this seqno, considering it is
unfeasible, it won't be of interest to the downstream peer. Note that if
the seqno request triggered a bump of the route seqno, the _first_ update
is always feasible.
Also expose a metric value to track how often we just skip route
selection.
Closes#329
Signed-off-by: Lee Smet <lee.smet@hotmail.com>
