Review PDFFastShutter code

[mrakitin]

https://github.com/NSLS-II-PDF/profile_collection/blob/b06e317cdc4734d46cd416528ef7bd8e7ab16a69/startup/15-optics.py#L55-L89

@tacaswell writes:

it looks like self._st is never not None

[mrakitin]

@awalter-bnl pointed out that the shutter re-work in NSLS-II/nslsii#52 will handle the states better.