synchronous run and resume functionality + nbrun for runner API

[madhur-ob]

Should only be merged after #1732

This allows us to do stuff like:

    runner = Runner('../try.py', metadata="local")
    result = runner.run(alpha=5, tags=["abc", "def"], max_workers=5)

    while not result.run.finished:
        print('trying again...')
        result = runner.resume()

Also allows us to use run inside environments which already have a running event loop such as Jupyter notebooks.

[savingoyal]

'NBRunner'?