storage: add noop behavior for bounds setting in pebbleIterator

[sumeerbhola]

This is to compensate for the bug fix in Pebble which removes
the noop behavior from Pebble
cockroachdb/pebble#1073

Added a test that checks that the noop behavior is working,
the contract regarding slice stability is followed, and that
the bounds are correct.

Additionally, there are some minor tweaks involving removal
and addition of some defensive code. The additions are
related to the pebbleIterator.reusable field.

Release justification: Fix for high-severity bug in existing
functionality. The Pebble bug fix that needs to be compensated
here could lead to incorrect results from iterators, even prior
to the latest seek optimization in Pebble (though our existing
tests were not failing prior to that latest seek optimization).

Release note: None

[cockroach-teamcity]

This change is 

[jbowens]

Reviewed 3 of 4 files at r1.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @itsbilal, @nvanbenschoten, @petermattis, and @sumeerbhola)

---

pkg/storage/pebble_iterator.go, line 197 at r1 (raw file):

			}
		}
		if !boundsChanged && upperBound != nil {

if upperBound != nil {

[sumeerbhola]

TFTR!

Reviewable status: complete! 0 of 0 LGTMs obtained (and 1 stale) (waiting on @itsbilal, @jbowens, @nvanbenschoten, and @petermattis)

---

pkg/storage/pebble_iterator.go, line 197 at r1 (raw file):

Previously, jbowens (Jackson Owens) wrote…

if upperBound != nil {

The lower bound may have changed, which means we don't need to look at the upper bound. I've added a code comment.

[sumeerbhola]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)