This commit teaches the storage schema APIs how to track statistics
and make them available via the returned `cursors.StringIterator`.
Statistics are only tracked when decoding TSM blocks or when scanning
the in-memory cache.
Closes#13541
The TimeRangeIterator permits linear or random index scans and
can answer whether the current key has data for the specified time
interval, considering any tombstones.
When there are no tombstones there are some opportunities for
optimization to skip decoding blocks. Specifically, if the
queried time interval overlaps any boundaries of the TSM index entries.
Stuart Carnie