ITS#7027 Implement priority/weight for DNS SRV records

ITS#7027 Implement priority/weight for DNS SRV records

From RFC 2782:

  A client MUST attempt to contact the target host with the
  lowest-numbered priority it can reach.

This patch sorts the DNS SRV records by their priority, and
additionally gives records with a larger weight a higher probability
of appearing earlier. This way, the DNS SRV records are tried in the
order of their priority.