onTicketBacklog
Called once the DocumentsWriter ticket queue has a backlog. This means there is an inner thread that tries to publish flushed segments but can't keep up with the other threads flushing new segments. This likely requires other thread to forcefully purge the buffer to help publishing. This can't be done in-place since we might hold index writer locks when this is called. The caller must ensure that the purge happens without an index writer lock being held.