[TACHYON-148] Fix bug in tachyon.examples.Performance

Fix the bug mentioned in [TACHYON-148]. The tachyon.examples.Performance class has two bugs:
Read test checks the data in different byte order with the write test.
In write test, it will repeatedly write the same file when one thread needs to write several files.
In this page (http://tachyon-project.org/master/Startup-Tasks-for-New-Contributors.html) it says that should be `i ++`.
But in the latest formatter (http://tachyon-project.org/master/resources/tachyon-code-formatter-eclipse.xml) it seems to be `i++`.
This made me confused.