release-22.1: kv: Limit concurrency of catchup scans.

[blathers-crl]

Backport 1/1 commits from #77725 on behalf of @miretskiy.

/cc @cockroachdb/release

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Limit concurrency of the catchup scans.
Prior to this change, distsender would issue all of the rangefeed
calls immediately, and the the KV server deal with limiting
the number of concurrent catchup scans it can handle. This has
a potential of using up entirety of the KV server budget by a single
rangefeed client (e.g. changefeed), thus starving all other users.

This PR adds a kv.rangefeed.catchup_scan_concurrency setting
to limit the number of catchup scans started by rangefeed client.

Fixes #77690

Release Notes (enterprise): Limit the number of concurent catchup scan
requests issued by rangefeed client.

Release Justification: Important stability and scalability improvement
to prevent a single rangefeed client from consuming entirety of catchup
scan iterator budget on a KV server.

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Release justification:

[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