job queue and async oplog-import/export (#395)

- Feature: New http endpoint for background jobs `/admin/jobs/status` which will return a response listing the currently active background jobs and their status
- Feature: New http endpoint for background jobs information `/admin/jobs/status/{jobid}` which will return a response detailing status, pending messages and progress status
  - GET will return a response detailing status, pending messages and progress status
  - DELETE will mark the job for cancelling and return without waiting for completion
  - If status returned is "Complete" or "Aborted" the jobid will be removed from the server and can not be queried again
- Feature: New zen command `jobs` to list, get info about and cancel background jobs
  - If no options are given it will display a list of active background jobs
  - `--jobid` accepts an id (returned from for example `oplog-export` with `--async`) and will return a response detailing status, pending messages and progress status for that job
  - `--cancel` can be added when `--jobid` is given which will request zenserver to cancel the background job
- Feature: oplog import and export http rpc requests are now async operations that will run in the background
- Feature: `oplog-export` and `oplog-import` now reports progress to the console as work progress by default
- Feature: `oplog-export` and `oplog-import` can now be cancelled using Ctrl+C
- Feature: `oplog-export` and `oplog-import` has a new option `--async` which will only trigger the work and report a background job id back

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