Gracefully reject concurrent requests from OSB clients (such as K8S svcat)

[gberche-orange]

Requirements

As a service operator,

• in order to support K8S clients that trigger concurrent update/get/delete requests without concurrency checks (as OSB mandates but CF currently prevents)
• I need concurrent updates/deletes to be rejected with a useful error message

Design outline

This is a refinement of #40

Steps

• Refine exiting endpont such as PATCH /v2/service_instances/:instance_id
  • Apply concurrency validation
    • Reject request if there is an ongoing update request pending
    • Reject concurrent update requests (OSB states it's broker responsibility to detect concurrency and reject concurrent requests)

Details of concurrency validation:

• Reject request if there is an ongoing update request pending
  • Before accepting a new update, compare (D)esired state and (A)ctual state. If D!=A then reject a concurrent update
• Reject update halting requests, Clarify what happens when deleting during a provision/bind request openservicebrokerapi/servicebroker#581
  • Compare (D)esired state and (A)ctual state. If D!=A then delete request
• Reject concurrent update requests (OSB states it's broker responsibility to detect concurrency and reject concurrent requests)
  • Rely on git to conflict on the epoq value with different timestamps

[gberche-orange]

Note that following introduction of osb-cmdb, all OSB requests are now originated from cloudfoundry, which currently ensures serialization of requests

[gberche-orange]

closing as won't fix since osb-cmdb protects coab from this scenario