[columnar] Adds columnar.vacuum() and columnar.stats()

[JerrySievert]

Adds two new UDF's:

• columnar.vacuum()
• columnar.stats()

The vacuum does an in-place vacuum, combining stripes and moving them to the beginning of the table space. This builds off of work done by @mkaruza.

The stats are useful for testing and getting information about the current state of the table and its stripes.

Note, this includes fixing an issue that @mkaruza found, as it affected this as well. As such, it changed the results of the columnar_vacuum test. (edit: those changes have been merged separately)

Given that this makes modifications to stripe data and metadata, as well as a change to an existing test, it needs a thorough review.

The vacuum UDF can optionally include a count of the number of stripes to modify, allowing for incremental vacuuming. It returns the number of stripes modified. When this number hits 0, then all vacuuming is complete.

CREATE TABLE t1 (a INT);
INSERT INTO t1 SELECT generate_series(1, 1000000, 1);
DELETE FROM t1 WHERE a % 3 = 0;
SELECT * FROM columnar.vacuum('test'::regclass, 1);

[wuputah]

Deferring to @mkaruza for review.

[mkaruza]

lgtm