hs: Lookup failure cache when introducing to an intro point

It turns out that if a descriptor contains 10 times the same intro point, and that the first introduction attempt fails, we'll try to connect to the same failing intro point again for all subsequent remaining intro points.

The intro point failure cache was introduced to avoid such a situation but it is only used between two descriptors that is if an intro point failed from the first descriptor and that intro point is still present in the second descriptor fetched, we ignore it.

However, this situation is about the same intro point in the same descriptor. In normal circumstances, this can't happen but it is still allowed by the protocol.

One issue with this is that a malicious service would induce many circuits out of the client than necessary. This can be used, theoretically, for a client guard discovery attack.

This affects both v2 and v3.