Consider use of `Cache-control` headers

Some tasks, performed by the CLI such as those involved in cf push, involve starting a job, then polling for completion, or more generally for a change of state, until it's done.

If an intermediate shared (or private) HTTP cache presents out of date results, this can result in unpredictable behaviour by the CLI.

Has the use of adding a Cache-control: no-cache header to requests generated by the CLI been considered?

While I realize that a correctly operating shared cache should not normally cache requests that have an Authorization header (such as those generated by the CLI), a private cache is permitted to cache GET requests, so it might be good to be explicit that we're always looking for a non-cached result.