[azeventhubs] Prep for release, fixing tests

[richardpark-msft]

This is just a simple update for QA, no fixes and also pushing the changelog date forward since it got delayed.

• Added in some more tests for recovery. There were some incidents in the stress cluster but they turned out to be red herrings. They did reveal that some more granular tests should exist and now we have them for both link and connection level recovery.
• Fixed a bug in one of the stress tests. We kept getting an odd (but non-fatal) error as the test tore down (after passing!) about the connection idling out. Turns out it was accurate - the batch test is a little unusual in that it doesn't send events continually - it sends them once at the beginning of the test and the consumers just keep rewinding to pick them up again. So the producer client just needed to be closed after it's part in the test was done.

Fixes #19220

[richardpark-msft]

/azp run go - azeventhubs

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[richardpark-msft]

/azp run go - azeventhubs

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).