Refactor the code to make task functions simple functions

[sdispater]

This PR makes the manager.task() method a true decorator by not altering the decorated function (except the addition of a defer() method to it to replicate the previous behavior).

What this allows is the ability to start decorating existing functions without modifying their signature (no task_run anymore) and leave them unit testable without relying on the internals of cabbage.

This code came up when trying to address #3 and I think if we go the route of this PR this makes the implementation of test specific tools in cabbage mostly unnecessary.

To sum it up, we can now have something like this:

from cabbage import TaskManager

manager = TaskManager()


@manager.task(queue="default")
def add(a, b):
    return a + b


add(1, 2)  # 3

[ewjoachim]

This looks nice but, it means we can't access information on the current task anymore, right ? Like if we want to add the current task id in the log context or anything ?

(This may be solved later, I'm not against having an unstable API while we iterate)

[ewjoachim]

@mgu @k4nar @Evelf I'd like a second opinion

[mgu]

I share you concerns @ewjoachim (about accessing current task information).
Nevertheless I like what has been made here 👍

[ewjoachim]

Ok, let's merge it then, and we'll see later how we want to access current task info.

Maybe we will add a parameter to the task decorator to decide if we want to pass this or not... In a later PR :D

[ewjoachim]

Hoping it's ok with everyone, I'll merge now, because I'd like to have a go at some tickets today.

[k4nar]

LGTM