Reduce operator (and others) may be quadratic with deep history #618
Description
We have reports from the field that when provided with many fine grained updates, even to a collection that has a small working set. Looking at the code, it seems like there are possible moments where this could happen, for example we build a CursorList of available batches, but navigation through a cursor list will be quadratic with the number of batches linked up (they were intended for LSM shaped data). This feels like a suboptimal way to handle large numbers of batches, especially if it expected that they will soon be (or already have been) merged.