'get next' steals request that is already assigned

We introduced this bug ourself because we added the assigned state, and is caused here: https://eipdev.ewi.tudelft.nl/gitlab/EIP/queue/blob/development/src/main/java/nl/tudelft/ewi/queue/service/RequestService.java#L57

What 'get next' currently does: if there is a request assigned to me, pick that one. else if there is a request that is queued, pick that one.

However, a request is also considered 'queued' when it is assigned to any TA. Therefore, if no request is assigned to you, there is a (high) probability that 'get next' steals a request that was assigned to another TA.

I think we want to replace the queue variable with a variable pendingRequests (which contains only requests with the status PENDING), and then change the code to do: if there is a request assigned to me, pick that one. else if there is a request in pendingRequests, pick that one.