add BufferedDeletes; separate DeleteQueue
separate flush control and stall control
separate and enrich IndexFileDeleter; simplifier several method signature by removing error return
implement IndexFileDeleter.checkpoint()
fix dead loop
implement IndexWriter.MessageState()
implement IW.segmentToString()
separate and implement around DWPT pool
add missing Close() to MergePolicy impls
fix deadloop
fix typo
separate config and live config
separate BufferedDeleteStream
separate ReaderPool
fix import
fix log typo
fix logging
move closing&closed to ClosingControl