Supporting byPassMergeSort shuffle for SGX

[pgaref]

Addressing #15

• Introducing new SHUFFLE_MAP type for bypassMerge sort shuffle
• Adding numOfPartitions as part of SGX RDD (needed by the shuffle partitioner)
• Introducing SGX partitioner - hash-based for now (to be changed by any pseudorandom function we want)
• Introducing sgxWrite interface as part of Shuffle writer, when SGX is enabled we use this path instead. The method takes as arguments both the encrypted records AND the record-partition mapping (recordMapping) as returned by the enclave worker
• Handling SHUFFLE_MAP_BYPASS task type in SGX worker - return a record-partition mapping iterator
• ShuffleMapTask checks if SGX is enabled and calls the appropriate shuffle interface
• ByPassSort shuffle test case with multiple partitions