MEDIUM: resolvers: replace bogus resolv_hostname_cmp() with memcmp()

MEDIUM: resolvers: replace bogus resolv_hostname_cmp() with memcmp()

resolv_hostname_cmp() is bogus, it is applied on labels and not plain
names, but doesn't make any distinction between length prefixes and
characters, so it compares the labels lengths via tolower() as well.
The only reason for which it doesn't break is because labels cannot
be larger than 63 bytes, and that none of the common encoding systems
have upper case letters in the lower 63 bytes, that could be turned
into a different value via tolower().

Now that all labels are stored in lower case, we don't need to burn
CPU cycles in tolower() at run time and can use memcmp() instead of
resolv_hostname_cmp(). This results in a ~22% lower CPU usage on large
farms using SRV records:

before:
  18.33%  haproxy                   [.] resolv_validate_dns_response
  10.58%  haproxy                   [.] process_resolvers
  10.28%  haproxy                   [.] resolv_hostname_cmp
   7.50%  libc-2.30.so              [.] tolower
  46.69%  total

after:
  24.73%  haproxy                     [.] resolv_validate_dns_response
   7.78%  libc-2.30.so                [.] __memcmp_avx2_movbe
   3.65%  haproxy                     [.] process_resolvers
  36.16%  total