Add query cache to ServerDataSource

[uap-universe]

It looks like the ServerDataSource could use a cache to remember the last query parameters s.t. subsequent calls to

• getAll()
• getElements()
• getFilteredAndSorted()

can return the last result without issuing a new request. On the other hand, there needs to be a way to invalidate this "cache", if you want to obtain fresh data from the server without changing the filter, page, or sorting.

I would override the refresh() method for this. This way you get new data from the server

• every time the query changes (changing the page or the filter)
• on explicit calls of refresh()