Add an async dns resolver.

[azjezz]

the DNS api should probably use the HH\Lib\Network\DNS namespace.

[fredemmott]

We're most likely to release our internal C++ async DNS client and HHVM extension (FB: T40713253), with non-HSL-appropriate naming.

When parts of the HSL start moving into HHVM, we'll likely stop wrapping existing builtins and make the HSL APIs native instead.

[fredemmott]

It might be more practical to:

• move FB to libevent2 (which should happen anyway)
• use libevent2’s DNS implementation

libevent2 also has better support for resolv.conf