Support async binding operations

[mattmcneeney]

Binding to an instance may encapsulate a long running workflow, similar to provisioning. Discussions in issue #137 have resulted in:

• Changing the Binding endpoint to support asynchronous operations.
• Adding a new last_operation endpoint for bindings (GET /v2/service_instances/:instance_id/service_bindings/:binding_id/last_operation)

Feedback welcome as always.

[cfdreddbot]

Hey mattmcneeney!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.

[mattmcneeney]

Issues this PR does not address:

1. How long should service brokers maintain the result of a binding operation? If the last_operation binding endpoint is called multiple times, should the result (ie. credentials) always be returned?
2. Would it make more sense to use a new endpoint to fetch information on bindings (see proposed spec) once the last_operation says the operation has completed?

I would propose that we review this change to the spec first as this brings binding in line with provision, and then we can discuss adding additional endpoints to fetch per-service-instance or per-binding information (eg. credentials, health, etc).

[duglin]

Can you rebase and squash?

[mattmcneeney]

@duglin done, but be aware the PR contains a few "tidy-ups" of various double spaces and unnecessary #s. I can go through and split these out if everyone would like (although that would be painful).

[mattmcneeney]

Closed as we will continue down this path on the back of #159 where GET endpoints are being added for service instances and bindings.