Update max_dl_per_request for IPv6 address length
max_dl_per_request depends on the maximum length of a textual IPv4 address.
To query IPv6 directories, the NS and MICRODESC calculations need to be updated for the maximum length of a textual IPv6 address in a URL. (That is, no brackets.)
While we're doing that, it would be nice to add a comment with the microdescriptor length calculation as well.