Allow OpacApi functions to return intermediate results

[johan12345]

E.g. if the API is waiting for some slow AJAX requests for non-essential data (e.g. search result status) to load, it could already pass an intermediate result to the app, so that it can be displayed more quickly.

Our current idea how to implement this is to add an optional callback as a parameter to search (and other functions, such as searchGetPage and account) that allows the API to pass an "intermediate result" - so in this case, a SearchRequestResult without statuses - and then give the final result as the return value of the search function once all statuses are fetched.