Update Worker to support concurrent predictions
This adds a max_concurrency parameter to Worker that allows it to accept
predictions while others are already in-flight.
I have removed the PROCESSING WorkerState; in concurrent mode, there is no
distinction between READY and PROCESSING because we might be able to accept a
prediction in either case.
Worker now keeps track of multiple in-flight predictions in a dictionary, keyed
on tag. Tags are required if max_concurrency > 1. Otherwise tags are
optional (and, if omitted, we store the prediction with tag=None).
There is one awkward place which is _prepare_payload(). As I understand it,
this synchronously downloads URLFiles, which will block us from processing any
other updates from the child while we download the URL.
