[master] Improve performance by avoiding quadratic behavour in web_saveDataSet
As strings are immutable in Python this method has quadratic runtime performance and becomes unusable with more than ~5,000 LFNs due to the wasted time on copying strings. By using a list and concatenating them once this can handle about 80,000 LFNs per second.
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*BookkeepingBrowser FIX: Significantly improve performance in web_saveDataSet
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Edited by Chris Burr