This patch addresses two cache issues

[sundeepn]

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1. 'cache tablename' directive overwrites existing memory tables and can cause memory leaks when the cache command is used on an already cached table. caching new partition also causes a memory leak if the parition already exists.
2. using 'cache' directive on a partitioned table causes table to be corrupt. Only the last cached partition is accessable with the rest being leaked.

[sundeepn]

@rxin - Please review.

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[harveyfeng]

#2 is actually a bug in SparkLoadTask#loadPartitionedMemoryTable(), I'll submit a fix in a bit

[harveyfeng]

This should address it: https://github.com/amplab/shark/pull/296

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