Resolver: fixed handling of partially resolved SRV.

Resolver: fixed handling of partially resolved SRV.

The resolver handles SRV requests in two stages.  In the first
stage it gets all SRV RRs, and in the second stage it resolves
the names from SRV RRs into addresses.

Previously, if a response to an SRV request was cached, the
queries to resolve names were not limited by a timeout.  If a
response to any of these queries was not received, the SRV
request could never complete.

If a response to an SRV request was not cached, and some of the
queries to resolve names timed out, NGX_RESOLVE_TIMEDOUT was
returned instead of successfully resolved addresses.

To fix both issues, resolving of names is now always limited by
a timeout.