hs-v3: Make service pick the exact amount of intro points

When encoding introduction points, we were not checking if that intro points
had an established circuit.

When botting up, the service will pick, by default, 3 + 2 intro points and the
first 3 that establish, we use them and upload the descriptor.

However, the intro point is removed from the service descriptor list only when
the circuit has opened and we see that we have already enough intro points, it
is then removed.

But it is possible that the service establishes 3 intro points successfully
before the other(s) have even opened yet.

This lead to the service encoding extra intro points in the descriptor even
though the circuit is not opened or might never establish (#31561).

Fixes #31548

Signed-off-by: David Goulet <dgoulet@torproject.org>