[scheduler] Rename future abstractions

[iyzhang]

The names of some of the abstractions in the scheduler are a bit confusing from Demikernel's point of view and they do not reflect our needs. First, we've renamed Future to Coroutine (even though they are not strictly coroutines). This is our basic unit of computation for performing I/O. Next, this PR creates a Task abstraction to replace the previous FutureResult abstraction. The scheduler uses Tasks for scheduling now instead of opaque Futures as before. Thus, you must wrap a Future/Coroutine in a Task before you can insert it into the scheduler because the scheduler no longer knows about generic Futures. This means that we are able to have a better view into what coroutines are running (and our background coroutines are also wrapped in Task, rather than being directly scheduled). This addresses issue #529