db: Add IngestShared test to exercise shared ingestion

[itsbilal]

This is a test-oriented change that adds more end-to-end testing for
ingesting shared sstables and also reading keys from them.

[cockroach-teamcity]

This change is 

[itsbilal]

Rebased this now that #2538 has merged.

[jbowens]

Reviewed 1 of 16 files at r2, all commit messages.
Reviewable status: 1 of 16 files reviewed, 1 unresolved discussion (waiting on @itsbilal and @RaduBerinde)

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testdata/ingest_shared line 482 at r2 (raw file):

  000004:[a@3#10,SET-c@9#10,SET]
5:
  000005:[b#2,RANGEDEL-d#2,RANGEDEL]

worth a test that exercises truncating the rangedel to the ingest sst bounds, ensuring it doesn't delete keys that fall within the physical sst bounds.

[itsbilal]

TFTR! Rebased to work with the newest changes.

Reviewable status: 0 of 16 files reviewed, 1 unresolved discussion (waiting on @jbowens and @RaduBerinde)

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testdata/ingest_shared line 482 at r2 (raw file):

Previously, jbowens (Jackson Owens) wrote…

worth a test that exercises truncating the rangedel to the ingest sst bounds, ensuring it doesn't delete keys that fall within the physical sst bounds.

We actually don't support that, as we force virtual sstable bounds to fall in places where there are no range keys/deletes:

pebble/internal/keyspan/truncate.go

Line 69 in 6746f41

panic("pebble: bounds should not be truncated")

But on closer thought, we might need to revisit this; seeing as we could defragment [a,b) and [b,c) as [a,c) even if b is a crdb range boundary and we'd need to be able to create virtual sstable bounds at b.