Fix the bug mentioned in [TACHYON-66]

Fix the bug mentioned in [TACHYON-66] "The memory usage statistics is
not right when multi-user cache the same file and stops their
connection".
Change the synchronized range
Add a unit test for cacheBlock

Add a unit test for cacheBlock
Merge remote-tracking branch 'upstream/master' into TACHYON-66
Update the UT to be concurrent

Update the UT to be concurrent and merge with the latest master branch.
Update the test code

Update the test code
Merge pull request #354 from PasaLab/TACHYON-66

[TACHYON-66] Fix the bug of the memory usage statistics in tachyon.worker.WorkerStorage