Fix tests for testStartVoldemortCluster to not consume so much memory

test/common/voldemort/ServerTestUtils.java
- remove stress test from normal testing
- test the method startVoldemortCluster once and confirm it returns a non-null Cluster object

test/common/voldemort/ServerTestUtils.java
- clarified a comment
Added the ability to delete old checksum files in the Build and Push reducer. Also updated the hadoop core library to version 1.0.4-p2-rc2 containing the fix to the User ID command usage.
Updated the hadoop-core jar to 1.0.4-p2