Integrity verification

[ankurdave]

Integrity verification would require the following changes:

1. Construct a task DAG after a job has been submitted using Spark's EventListener API.
2. Serialize the DAG using Flatbuffers.
3. Construct task identifiers within each enclave call based on the operation that the task is to perform.
4. Ensure that all output from an enclave call is uniquely identified with its task identifier.
5. Ensure that each enclave call records the identifiers of each of its inputs.
6. Send these (input, task, output) tuples from each enclave call to a verifier enclave.
7. Implement the verifier enclave, which takes the task DAG and the set of (input, task, output) tuples, and checks whether the two are isomorphic.