1. MemTableFlushExecutor maintain a ThreadPool to receive FlushTask.
2. FlushToken is used to seperate different tasks from different tablets.
Every DeltaWriter of tablet constructs a FlushToken,
task in FlushToken are handle serially, task between FlushToken are
handle concurrently.
3. I have remove thread limit on data_dir, because of I/O is not the main
timer consumer of Flush thread. Much of time is consumed in CPU decoding
and compress.