fixed 2 bugs and made clearer that Long-term timer in DnsResolver is not affected...

fixed 2 bugs and made clearer that Long-term timer in DnsResolver is not affected by finalize_resolve

bugs:
* HostAddress::is_valid returned opposite
* use the returned IP to ping and do not request another in PingScheduler::ping
* move reset of ContinueOnOutdatedIp within PingScheduler to after ping

long-term timer:
* added a variable DnsResolver: LongtermTimerIsActive
* added a function to ResolverBase: is_waiting_to_resolve
* added a function cancel_resolve to PingScheduler that decides whether to cancel or not
* added an arg to DnsResolver::stop_trying