2.3

• Delete excluded file thread from thread pool 7b73735
  Now, a thread optimizer may run before each round of prefetch.
  If no "delete this file" record exist, the optimizer will not run.
  
  The performance result is before this version, I may generate 26968 thread in total,
  most of them are empty function, return immediately, only thread overhead.
  After this version, only 155 thread fired in the second round of prefetch.
  
  However, don't expect huge performance gain:
  on my personal PC (E3-1231-V3), no noticeable performance different shows in benchmark result.
  This update may only try not to abuse CPU.