We don't return early if arping was skipped. This can happen when
insertNeighbor() is invoked by the non-refresh path and nexthop is not
new.

Make sure that lastPing is updated only if arping was sent and it was
successful (if hwAddr != nil condition).

Signed-off-by: Martynas Pumputis <m@lambda.lt>

We can inspect the neighLastPingByNextHop map to check when
insertNeighbor() or deleteNeighbor() was called.

Signed-off-by: Martynas Pumputis <m@lambda.lt>

It's possible that in the case of multiple concurrent insertNeighbor()
executions the oldest (or older) goroutine will overwrite the latest
arping result due to the fine-grained locking.

To fix this, avoid updating neigh entry if we detect that prev last ping
timestamp is after our arping timestamp.

Signed-off-by: Martynas Pumputis <m@lambda.lt>