release-21.1: kvserver: throttle AddSSTable requests with IngestAsWrites

[erikgrinaker]

Backport 1/1 commits from #73904.

/cc @cockroachdb/release

---

Store.Send() limits the number of concurrent AddSSTable requests
and delays them depending on LSM health via Engine.PreIngestDelay, to
prevent overwhelming Pebble. However, requests with IngestAsWrites
were not throttled, which has been seen to cause significant read
amplification.

This patch subjects IngestAsWrites requests to Engine.PreIngestDelay
as well, and adds a separate limit for IngestAsWrites requests
controlled via the cluster setting
kv.bulk_io_write.concurrent_addsstable_as_writes_requests (default
10). Since these requests are generally small, and will end up in the
Pebble memtable before being flushed to disk, we can tolerate a larger
limit for these requests than regular AddSSTable requests (1).

Release note (performance improvement): Bulk ingestion of small write
batches (e.g. index backfill into a large number of ranges) is now
throttled, to avoid buildup of read amplification and associated
performance degradation. Concurrency is controlled by the new cluster
setting kv.bulk_io_write.concurrent_addsstable_as_writes_requests.

---

Release justification: prevents pathological performance degradation.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[erikgrinaker]

TFTRs! Mind giving #74071 a ✅ too?