Lower epsilon used for jitter verification from HBASE-15324

The current epsilon used is 1E-6 and its too big it might overflow the desiredMaxFileSize. A trivial fix is to lower the epsilon to 2^-52 or even 2^-53. An option to consider too is just to shift the jitter to always decrement hbase.hregion.max.filesize (MAX_FILESIZE) instead of increase the size of the region and having to deal with the round off.