dgram: add custom lookup function in sockets

This commit adds support for custom DNS lookup functions in dgram sockets. This is similar to the existing feature in net sockets.

Refs: https://github.com/nodejs/node/issues/6189

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

dgram