Implemented AtomicBoolean of havePendingRead in readMoreEntries()

[devinbost]

I noticed that we're performing a compareAndSet type of operation in readMoreEntries() in the persistent dispatchers on the field havePendingRead, but the way we're doing it isn't thread-safe since readMoreEntries() isn't synchronized.
As @rdhabalia mentioned in #9789 that there are no pending reads despite backlog and available permits, it seems like there may be a concurrency issue with the pending read status.

I need someone with more experience with concurrency primitives in Java to review this PR to indicate if it actually improves anything because I only found one place where there was a clear "compareAndSet" type of operation, so I don't see clearly competing accesses, and it's not clear to me if access on this field occurs from multiple threads in this location.

[devinbost]

@merlimat ?

[devinbost]

@lhotari Is this still needed now that #10413 has been merged?

[merlimat]

I think adding the sync block was the correct fix and should already be enough.