Timer unref option

[DullReferenceException]

Add option to unref the TTL timer. Doing so will avoid a cache keeping a NodeJS process alive with its timer event handlers.

[DullReferenceException]

The reason for this PR is that when I tried integrating it in our application, unit test processes and the worker processes as well were not exiting as they did before, and the setTimeout calls were the cause. The .unref() will tell node to ignore that there are outstanding timeouts when it's deciding whether the event loop can terminate. Mind taking a look, @norganna?