[improve][broker] Change bookkeeperClientSeparatedIoThreadsEnabled to True by default

[zhanglistar]

Motivation

Apache Pulsar PR #16333 introduced bookkeeperClientSeparatedIoThreadsEnabled so the BookKeeper client can use a dedicated IO thread pool instead of sharing the broker's Pulsar EventLoopGroup. The upstream default remained false for backward compatibility.

In our deployment, sharing IO threads between the broker and the BookKeeper client can cause contention under storage-heavy workloads. Enabling separated IO threads by default isolates BookKeeper network I/O from broker I/O and better matches our production expectations.

Modifications

• Set bookkeeperClientSeparatedIoThreadsEnabled=true in conf/broker.conf and conf/standalone.conf, and update related comments.
• Change the Java default in ServiceConfiguration from false to true, and update @FieldContext documentation.
• Update unit tests to reflect the new default:
  • ServiceConfigurationTest#testBookKeeperClientIoThreads
  • BookKeeperClientFactoryImplTest#testBookKeeperIoThreadsConfiguration

Operators who want the previous behavior can set:

bookkeeperClientSeparatedIoThreadsEnabled=false

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

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

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

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

[zhanglistar]

#19061 Not related flaky test.