add a test case indicating we reset the stats when starting a new fragme...

...nt

Don't merge. Also bypass the bug of ENG-6517 by using inequality checking instead of equality checking.

Still need to discuss on the test case which indicates the peak memory usage is larger than the "current" memory usage.
It is nice to run valgrind on this tests, there may be a difference on the memory.  Besides, this may also be an additional memory test for DELETE AS WE GO feature.

Good job!