revisit dns_resolver::impl::do_sendv() as c-ares now handles EWOULDBLOCK for UDP

[tchaikov]

quote from #2472 (comment)

Finally, I should mention this note is no longer valid in your source code:
seastar/src/net/dns.cc
Line 1017 in bddc7b2
// 1.) c-ares does not handle EWOULDBLOCK for

we always pretend that we are able to send the whole payload even when the promise of the write promise is not available immediately. seems this approach simplifies the implementation. but either way, according to Brad, the comment does not apply anymore.