Try to reallocate only reallocable circuits.

This patch solves a bug:
- With previous circuit selection strategy (select always the first circuit using given port) it may happen that the first one had no alternative route, and thus, fails to be re-routed in next steps. In that case, the circuit without alternative keeps being the first in the list and keeps being selected each time the re-route mechanism is triggered, although is not re-routed. This causes no harm if that circuit is the only one, but when other circuits are available, it may be the case that re-routing these others solves congestion. However, the strategy will never try these others :(

This patch solves this issue by picking up a circuit that is known to have an alternative route.
This way, each time the re-routing mechanism is triggered, a circuit is re-routed (if any available).