When set to "off", only IPv6 addresses will be resolved, and no
A queries are ever sent (ticket #2196).

It is safe because re-sending still works during graceful shutdown as
long as resolving takes place (and resolve tasks set their own timeouts
that are not cancelable).

Also, the new ctx->cancelable flag can be set to make resolve task's
timeout event cancelable.

The flag indicates that the resolve handler is called asynchronously after the
resolve function ngx_resolve_name()/ngx_resolve_addr() exited.

It keeps the actual state value of a DNS SRV subrequest
and can be used to report a more detailed log for
failed SRV records.

It hints the amount of time a response could be considered
as valid.

Resend DNS query over TCP once UDP response came truncated.

Previously, a global server balancer was used to assign the next DNS server to
send a query to.  That could lead to a non-uniform distribution of servers per
request.  A request could be assigned to the same dead server several times in a
row and wait longer for a valid server or even time out without being processed.

Now each query is sent to all servers sequentially in a circle until a
response is received or timeout expires.  Initial server for each request is
still globally balanced.

They will be used for TCP connections as well.