Improve logic to avoid block call under non blocking thread

[moarychan]

Description

Cherry-pick from https://github.com/Azure/azure-sdk-for-java/pull/40772

Issues #12500, #35845 and #35215 appear to be due to EventHubsTemplate.doSend performing a block() call. When used by the non-async methods in EventHubsTemplate, or the async methods when a non-parallel Reactor Scheduler is being used, this will work well enough. But when using a parallel Reactor Scheduler, blocking calls are prohibited.

This PR restructures the code in EventHubsTemplate.doSend to not use a blocking call. A test was added first to verify that we could detect this condition by performing one of the existing test cases with Scheduler.parallel(). After the changes to EventHubsTemplate, this test passes.

And fix the same issue for DefaultMessageHandler.handleMessageInternal.

All SDK Contribution checklist:

• [x] The pull request does not introduce [breaking changes]
• [x] CHANGELOG is updated for new features, bug fixes or other significant changes.
• [x] I have read the contribution guidelines.

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[azure-sdk]

API change check

API changes are not detected in this pull request.