Schedulers refactor

[syrusakbary]

This PR refactors the schedulers so it's easy to plug different instances, and removes the unnecessary abstraction Context.

Also, by default the event loop is deactivated (trampoline is disabled, so the promise resolution go directly to the scheduler).
Available schedulers:

• promise.schedulers.ImmediateScheduler: executes as soon as is called, good for testing or environments without event loop.
• promise.schedulers.AsyncioScheduler: scheduler using the asyncio event loop
• promise.schedulers.GeventScheduler: scheduler using the gevent event loop

How to use a custom scheduler?

from promise import set_default_scheduler
from promise.schedulers.gevent import GeventScheduler

set_default_scheduler(GeventScheduler)

Also, the dataloader will use the default scheduler for loading.

@phil303

Related issue: graphql-python/graphene#488

[nicwolff]

Is this still the recommended way to use gevent with DataLoader? I can't find any documentation on that, but with the code shown above I get

GeventScheduler.call() missing 1 required positional argument

on all requests.