Async DNS lookup support via libevent DNS module.

Async DNS lookup support via libevent DNS module.

With libevent 1.x we use evdns_base_resolve_ipv4() API,
which does not use /etc/hosts, but returns TTL.

With libevent 2.x we use evdns_getaddrinfo() API, which uses /etc/hosts
but does not return TTL.

As we need to have our own TTL handling anyway, the TTL is ignored
on libevent 1.x too, to be consistent.

New config variable 'dns_max_ttl', which sets time how long result
can be reused.