[improve] Do not process acks in the Netty thread

[dlg99]

Motivation

Cherry-picked from internal @eolivelli work.
Do not block netty threads if cursor has a lot of individuallyDeletedMessages or needs to compress the PositionInfo (separate change)

Modifications

Run acks on pulsar executor

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

NO

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

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