Updated some code comments in DefaultIOStream.cpp

There was a misleading TODO comment that encouraged to use fseek/ftell instead of fstat. However, fstat has been used intionally because fseek/ftell is potentially unsafe.
So I replaced the TODO and added some explanation why fstat is being used instead.
Merge pull request #196 from AIM360/UpdatedCodeComments

Updated some code comments in DefaultIOStream.cpp