Support SideEffect task that does not need any existing task.

[mchen07]

Currently, to fire and forget an async task that does not depend on any existing task, people are implementing this by using the ParSeq interface to return a Task.action that does nothing, and appending a side effect to it in order to spin off a thread that will by executed asynchronously.

We could improve API and provide Task.withSideEffect() that does not need any existing tasks to avoid boilerplate so:

Task.action(() -> { }).withSideEffect(response ->

Would become:

Task.withSideEffect(response ->

[jodzga]

This is a duplicate of #132