**Summary**
This PR adds a server state callback that is invoked whenever a query to
a DNS server finishes.
The callback is invoked with the server details (as a string), a boolean
indicating whether the query succeeded or failed, flags describing the
query (currently just indicating whether TCP or UDP was used), and
custom userdata.
This can be used by user applications to gain observability into DNS
server health and usage. For example, alerts when a DNS server
fails/recovers or metrics to track how often a DNS server is used and
responds successfully.
**Testing**
Three new regression tests `MockChannelTest.ServStateCallback*` have
been added to test the new callback in different success/failure
scenarios.
Fix By: Oliver Welsh (@oliverwelsh)
Brad House
Oliver Welsh