This class defines a threshold between ranges of clusterings. It can either be a start or end bound
of a range, or the boundary between two different defined ranges.
The latter is used for range tombstones for 2 main reasons: 1) When merging multiple iterators having range tombstones (that are represented by their start and end markers), we need to know when a range is close on an iterator, if it is reopened right away. Otherwise, we cannot easily produce the markers on the merged iterators within risking to fail the sorting guarantees of an iterator. See this comment for more details: https://goo.gl/yyB5mR. 2) This saves some storage space.