Fine-grained snapshot diffs

[Shillaker]

Previously snapshot diffs were only down to a page-level granularity, which meant that distributed threads modifying a shared array would trample on each other (as they would all have their own modified version of the same page).

This PR adds a couple of steps to the existing snapshot diff process:

• Push the snapshot to worker hosts before thread execution
• Map thread memory from the snapshot
• Reset dirty page checks
• Execute the thread
• Get dirty pages
• Perform a bytewise comparison of the thread's memory with the original snapshot
• Push the bytewise diffs back to the original host (instead of the page-wise diffs)

Changes:

• Add logic for working out bytewise diffs based on dirty pages
• Encapsulate all the batch-level variables in an executor inside an ExeutorTask class (instead of a mix of a std::pair and class members on the Executor)
• Move logic related to applying diffs into the SnapshotData class
• Share test code via SnapshotTestFixture
• Check for zero-size snapshots (previously missing)

[csegarragonz]

LGTM, very minor couple things.