Add better handling for pitstop delays

[nelliemckesson]

Currently, if pitstop doesn't process the file within a certain amount of time, bookmaker assumes there is an error. We should add better handling for this, working off of the pitstop log or something.

[nelliemckesson]

We should make the pitstop handler a standalone process. When the PDF gets sent to pitstop, it should generate it's own "Pitstop_processing_please_wait" file, and then remove that when pitstop is done. We need to figure out how to read the pitstop log to know when the file is done, instead of just continuously pinging it, and so that we can return an error if pitstop does indeed fail.