server_name: tweak branch order for performance

[djc]

I was playing a bit with the layout of the core loop, and this seems like a nice improvement:

Benchmarking dns_name/typical: Collecting 100 samples in estimated 5.0004 s 
dns_name/typical        time:   [112.08 ns 112.43 ns 113.02 ns]
                        change: [−22.891% −22.334% −21.810%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 12 outliers among 100 measurements (12.00%)
  7 (7.00%) high mild
  5 (5.00%) high severe
Benchmarking dns_name/ip_like_rejected: Collecting 100 samples in estimated 
dns_name/ip_like_rejected
                        time:   [39.069 ns 39.130 ns 39.190 ns]
                        change: [+26.328% +29.896% +35.307%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 6 outliers among 100 measurements (6.00%)
  1 (1.00%) high mild
  5 (5.00%) high severe
Benchmarking dns_name/max_length: Collecting 100 samples in estimated 5.0000
dns_name/max_length     time:   [132.06 ns 133.17 ns 134.80 ns]
                        change: [−25.399% −24.763% −24.139%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
  2 (2.00%) high severe

This seems like a good trade?

(I experimented with a few other ways to order the branches but this seems optimal.)