Make FaceNet pickalable

In this MR FaceNet was made picklable. Two things were necessary.

1. Organize the initialization of the public variables of the facenet class
2. Add a semaphore to control two critical regions (call and getstate). This allows Dask to smoothly do multithreading.

LGTM but still uses the old tf1 API.

merged

mentioned in commit c592bf92