Fffmpeg does not scale.

[tomp736]

When processing media to generate thumbnails FfmpegService retrieves all available media. If multiple instances of pymp_server exist that are designated the role of processing media, they are not aware of who is processing what, and will all potentially (usually) process the same media.

The expected behavior is that when scaling this service, it will allow for media to be processed faster by means of splitting the work across the instances with the FfmpegService.

[tomp736]

Implement Queue using redis lists with lpop and rpush.

https://redis.io/docs/data-types/lists/

• add a new redis data access class (RedisMediaProcessQueue) that uses a redis list along with rpop and lpush methods implemented. The list elements will be media_info objects.
• during the existing scan that is done by the media_registry service, it will push media_info items onto the queue if they need to be processed (meaning, the media has no thumbnail or metadata present).
• during the existing scan that is done by the ffmpeg service, it will pop media_info from the new redis list and process the media.