Optimize fetch calls for get element

[Syndesi]

Currently fetch calls only happen during get element calls, if the element is not already cached.
While this usually limits the amount of fetch calls to 1, it does not work if multiple get element calls for the same element happen close together, before any of the fetch calls are finished.

This might be resolved by keeping already started fetch calls/promises inside a cache, and returning already existing promises, if a match is found.