Make each perftest folder distributed in several folders

Currently nGrinder uses ${NGRINDER_HOME}/perftest for the perftest store.
If the tests goes up to 30,000, it will start to create new file. Because each folder in linux has the sub folder limitation.

= There is a limit of 31998 sub-directories per one directory, stemming from its limit of 32000 links per inode.

We need to separate each perftest stored in the different folder like

• perftest
  + 0000
  = 1~999
  + 1000
  = 1000~1999
  + 2000
  ....
  + 3000