Motivation:

The AsyncDNSResolver currently throws an error when there are no records
associated with a name. However, most query responses are arrays or
contain arrays. As such it's unclear to callers whether they should be
catching an error of a particular type or checking for the presence of
values some combination of the two.

Modifications:

- Modify the `DNSResolver` API to return `nil` where only a single value
  is expected (e.g. CNAME).
- Update documentation on `DNSResolver` to clarify the expected return
  values when no values are resolved.
- Update the c-ares backend to check for no data when parsing and return
  empty responses
- Update the DNSSD backend to pass in `nil` data to the parser when the
  status is timeout
- Remove the `noData` error code as it shouldn't be reachable anymore

Result:

Queries with no response either return empty or nil

Motivation:

The 'AsyncDNSResolver.Error' is closely modelled on the c-ares error
codes and doesn't fit well with the DNSSD error codes. Further, the
DNSSD backed doesn't map its error cdes back into an
'AsyncDNSResolver.Error' so all DNSSD errors become 'other' errors.

Modifications:

- Reduce the number of error codes in 'AsyncDNSResolver.Error' to a few
  high level error codes and a catch-all 'internal' error code.
- Update mappings from c-ares error codes, add mappings from dnssd error
  codes.
- Add a 'source' property, which can be 'any Error', allowing users to
  get fine grained error information if necessary.

Result:

Error codes are more consistent across implementations.