ZOOKEEPER-4271: Flaky test - ReadOnlyModeTest.testConnectionEvents

[amichair]

Some refactoring and cleanup, followed by the fix - client wasn't properly shut down between tests so occasionally they contaminated each other with unwanted events.

[maoling]

@amichair very very Sorry for our late, I'll take a look at next weekend(07-10)

[eolivelli]

I believe that this patch is in the good direction
In my opinion renaming CountdownWatcher to StateWatcher is making the patch a bit hard to review.

can you please revert that refactor ? this way we can focus on your change

[amichair]

I can undo the refactor with a bit of work, but the rename is in a separate commit to make it trivially easy to review... also, it no longer uses a countdown latch, so the name doesn't make sense. Also, it's an implementation detail where before the inner field was accessed directly from outside and is now encapsulated instead, so it makes sense to rename to what it does rather than how it (was) implemented... no?

[kezhuw]

Superceded by #1896.