Bookie should return to read-write mode once the disk usage drops before the threshold

Once a bookie goes above the diskUsageThreshold and becomes readOnly, it will never go back to readWrite even if the disk usage drops significantly.
When the LedgerDirsMonitor gets a NoWritableLedgerDirException, it will put the bookie in read only mode, but will also stop monitoring and never goes back to read-write mode when the disk usage drops below threshold.

Solution:
The LedgerDirsMonitor thread should not break out of the while loop when it gets NoWritableLedgerDirException, and should continue monitoring the directories.