This adds an lru cache for the columns that are produced as tags. When
producing the columns that are part of the group key, it will generate
the column and then keep it in an lru cache to reuse for future tables.
The start and stop column are effectively cached for every table because
they are special and will be the same for all of the tables.
For the tags, it retains the most recently used since they may be used
by a future table. That way most of the columns will get shared with
each other.
When the size differs, a slice is used so the underlying data is still
shared, but the size is different.
fe94c5cae4