Circuit Build Timeout needs to count hidden service circuits

The Circuit Build Timeout code currently ignores all circuits with a desired_path_len other than 3. This causes it not to count hidden service circuits of longer path lengths.

When we change the path selection for Proposal247/#9001 (moved), we are going to want to count those circuits, because we want the circuit build timeout to prune 20% of Prop247 circuits, too. If we omit those circuits from the CBT calculation, we risk timing out either too few, too many, or even all of them.

The simplest way to fix this is to change circuit_timeout_want_to_count_circ() to decide to count every circuit once it has completed 3 hops, even if it plans to build more.

We may also want to alter the timeout application in circuit_expire_building(), depending on the prop247 implementation we choose. In some versions of the proposal, we can end up creating what are technically 5 hop paths (though these topologies were not very popular in Wilmington).