Support checking whether a submission currently exists in the queue for a particular piece of strategic metadata #64
Description
In certain use-cases it can be useful to check whether a submission for that particular situation already exists. (And if it does, Producer code might for example decide to keep waiting for that submission to finish, or instead to cancel that one and start a new one).
Instead of requiring people to store that information separately from Opsqueue, we already have the strategic metadata available.
So the goal of this task is to introduce a new function to the ProducerClient, which you can give one (or multiple, if it isn't too much effort) key-values of strategic metadata, which returns a list of in-progress submissions matching that set of metadata.
For example, you could ask whether there are any submissions for user_id: 42.