Fix O(n^2) performance when parsing a big pile of extrainfos

We were doing an O(n) strlen in router_get_extrainfo_hash() for
every one we tried to parse.  Instead, have
router_get_extrainfo_hash() take the length of the extrainfo as an
argument, so that when it's called from
extrainfo_parse_from_string(), it doesn't do a strlen() over the
whole pile of extrainfos.