Datastore load testing

[vidartf]

This is a split off from an initial discussion in #410. The issue is about the growth of element id lengths for large random-location inserts in sequences (lists, text).

[vidartf]

Some points I think it would be nice to have answered:

• How likely is this issue to affect "normal" use (smaller edits) for RTC?
• How severe is it? E.g. when the issue appears, can the user safely halt the collaboration, then restart it without loss of data?
• What mitigation routes exist? @sccolbert indicated that an optimized way of using id spans would help alleviate this.

[sccolbert]

Something that doesn't make sense is the random inserts causing the id lengths to increase more than appending to the end of the document. I would expect the opposite behavior.