Concurrent Metadata Writing

[lehitoskin]

Consider replacing the current image-path-hash-thread with semaphores.

Issues that come to mind:

• This probably isn't something I should bother with outside of wanting idiomatic(?) racket
• Changes to the metadata must be queued and done in order
• Is having them be threads necessary in the first place? It might be even easier/better to just have a queue instead of a hash that executes the metadata writing.

In the Racket Guide, the initial example has semaphores for threads that print, but the order of the threads that execute appear to be in reverse order of creation.

[lehitoskin]

The semaphore example in the guide gives inconsistent results. If change is going to happen, a queue might be the better route.