Performance improvements to -isNumberMatchingDesc:numberDesc:

Summary:
This was taking quite a bit of time at startup. This improves the performance of -hasValue:
greatly by doing a non-whitespace check more directly, and improves the performance of
-matchesEntirely:string: slightly by caching regex strings.

Test Plan: ran app; didn't crash. Would like some guidance here

Reviewers: alon

Subscribers: reviews-photos

Differential Revision: https://tails.corp.dropbox.com/D40936