*: simplify pointCollapsingIter, use vanilla iters for foreign SSTs

[itsbilal]

Previously we were using pointCollapsingIters to collapse obsolete points in foreign sstables. This was necessary because of snapshots that were open at the time those sstables were written, however they really blew up code complexity and performance. This change transitions to using vanilla sstable iterators for foreign sstables with hideObsoletePoints set to true. We error out if the sstable format for a foreign sstable is not high enough to support the obsolete flag.

Also remove all cases in the pointCollapsingIter that were not used by ScanInternal (which is the last remaining user of it).

Part of #2756.

[cockroach-teamcity]

This change is 

[itsbilal]

Enjoy the code deletions, y'all

[RaduBerinde]

Reviewable status: 0 of 5 files reviewed, all discussions resolved (waiting on @sumeerbhola)

[itsbilal]

TFTR!

[sumeerbhola]

Reviewable status: 0 of 5 files reviewed, 1 unresolved discussion

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table_cache.go line 523 at r2 (raw file):

	}

	if provider.IsForeign(objMeta) {

(tardy drive-by comment)
this could use a code comment with justification and a possible optimization. Something like:

// A foreign sst is assigned a seqnum s when it is ingested into Pebble. Only non-obsolete point keys in that sst are potentially relevant to an iterator, and these non-obsolete keys have seqnum s, and are only relevant if opts.snapshotForHideObsoletePoints > s. So we can always set hideObsoletePoints=true. Additionally, we could optimize by returning emptyIter when opts.snapshotForHideObsoletePoints <= s.

[sumeerbhola]

Reviewable status: 0 of 5 files reviewed, 1 unresolved discussion

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table_cache.go line 523 at r2 (raw file):

Previously, sumeerbhola wrote…

(tardy drive-by comment)
this could use a code comment with justification and a possible optimization. Something like:

// A foreign sst is assigned a seqnum s when it is ingested into Pebble. Only non-obsolete point keys in that sst are potentially relevant to an iterator, and these non-obsolete keys have seqnum s, and are only relevant if opts.snapshotForHideObsoletePoints > s. So we can always set hideObsoletePoints=true. Additionally, we could optimize by returning emptyIter when opts.snapshotForHideObsoletePoints <= s.

Ignore that optimization comment:

• I remembered that we were still doing SeqNumL5 etc. when I saw internal/rangekey: remove ForeignSSTTransformer #2782 (review)
• Once we remove SeqNumL5 etc., the only "common" case where pts.snapshotForHideObsoletePoints <= s is when the iterator is being created using a (conventional) Snapshot. But we're breaking conventional snapshots in IngestAndExcise, so they will not be getting used. And EFOS will not typically see this case.