release-23.1: sql/pgwire: buffer messages during COPY TO and startup

[blathers-crl]

Backport 3/3 commits from #99761 on behalf of @rafiss.

/cc @cockroachdb/release

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fixes #97314
backport fixes #99546

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sql: buffer COPY OUT data

Rather than sending each COPY result row one-by-one, now the data will
get buffered, then flushed when the buffer size limit is reached or when
sending ReadyForQuery.

This fixes an issue that was causing the ruby-pg test to hang, since it assumes
the data will be buffered.

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pgwire: buffer startup messages when creating connection

This avoids sending each ParameterStatus one-by-one.

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sql: refactor CopyIn handling

This makes it so we don't need to manually send a CommandComplete.
Instead, when the CopyInResult is closed, CommandComplete will be sent,
similar to how it works for other message types.

Release note: None

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Release justification: contains a fix for a hanging test

[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?

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is