release-21.2: util/encoding: support deep copying when decoding bytes ascending

[blathers-crl]

Backport 1/1 commits from #70740 on behalf of @yuzefovich.

/cc @cockroachdb/release

---

There is an optimization when decoding bytes ascending when it might
return the decoded value that shares the same storage as the input
buffer. However, this optimization might not be desirable in all cases:
in particular, if the input buffer came from the large BatchResponse,
then the decoded value might keep that whole response from being GCed.
This commit adds a couple of methods to avoid using the shared storage
by always performing a deep copy. It also audits all callers of the
relevant methods on the KV-SQL boundary (the fetchers) to make sure they
perform the deep copy when needed.

Release note: None

---

Release justification:

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• 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 

[yuzefovich]

I think it's worth getting this in for 21.2.1.

[mgartner]

LGTM for 21.2.1.

[yuzefovich]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)