tor-chanmgr: update allowed pending channels

Following some discussion, we've decided to behave closer to C-tor and allow a wider range of pending channels for a given relay target. This should reduce the number of channels between the same two relays and allow for more circuit multiplexing on the channel, at the expense of possibly a longer delay finding a channel if the final channel ends up being not suitable for the target and it needs to retry.

Our channel selection code is still very minimal, so this MR mostly updates the comments to try to explain the reasoning behind the choices, or what choices we should make when we implement more of this logic.