DNS watchdog support implementation: a SER module or an external application can...

DNS watchdog support implementation: a SER module or an external application can inform the core that the DNS servers
are down. No DNS query is performed in this case, and even expired entries are used from the cache. Use
dns_set_server_state() function from modules or the dns.set_server_state RPC command to set the sate of the DNS
servers.

The feature is desabled by default, enable it with -DDNS_WATCHDOG_SUPPORT (required DNS cache enabled)