Fix gistkillitems & add regression test to microvacuum

Hi hackers.

While looking at [0] I noticed that XLOG_GIST_DELETE & XLOG_GIST_PAGE_DELETE
records are not covered.

This thread addresses XLOG_GIST_DELETE, which is also known as a
microvacuum feature.

test.sql contains regression test that trigger this code to be
exercised in stream_regress.pl TAP test.

Test is as follows: we create a gist index on the table, then we
insert exactly 407 records, making the root page full (next insert
will trigger page split). Then I delete all tuples from relation and
trigger Index Only scan to do kill-on-select (killtuples). It marks
gist 0 page (which is root and is leaf) as has_garbage. Then, the next
insertion triggers xlog_gist_delete record.

To verify this I use pageinspect and pg_waldimp (locally). Also this
test is dependent on block size being 8192 which is not good.

And all of this does not work actually without v1-0001, because there
is a bug in GiST which does not call gistkillitmes for the very first
(root) page.

There is also test2.sql which inserts a single tuple, not 407. It can
be used to verify v1-0001.

[0] coverage.postgresql.org/src/backend/access/gist/gistxlog.c.gcov.html

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Best regards,
Kirill Reshke