The priority field in an SRV RR indicates a preferential order at which
the underlying targets should be utilised. We need to prefer healthy
services in order of priority, where 0 is highest.
Prior to this commit, we relied on whatever order the
dnsclient.getresources method returned. As it turns out, this assumption
is incorrect. The order returned is likely whatever order the system
resolver received DNS responses in, which may not be ordered according
to the spec.
This introduces a ResolvedAddress type which holds the priority value
for SRV targets, or a stand-in priority of zero for A/AAAA RRs. This
type is used as a return value from the underlying name resolution
routines in Name and SRVName.
In this manner, all ordering by priority and resolved time can be
performed directly within the ResolverCache class and calling code can
continue to be none-the-wiser.
Before sorting, we still ensure that we only consider targets with a
priority within the given threshold as previously implemented.
See t/115911.
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