Investigate stripedReadThreadpoolSize's impact on the performance of UnEC actions

[PHILO-HE]

The default value for stripedReadThreadpoolSize is 18, which can be easily reached for large number of files. If no thread is available in the thread pool, the read task will be rejected but be executed by the current thread and a log is reported: "StripedRead- task is rejected by ThreadPoolExecutor. Executing it in current thread." The impact on performance will be investigated. Maybe the property should be set according to "smart.cmdlet.executors" and EC schema.

[PHILO-HE]

I noticed that a method called initThreadsNumForStripedReads in DFSClient can be used to set threads num in this scenario.