VerifyFileStore: Check if two FileStore's match after applying a set of operations.

VerifyFileStore: Check if two FileStore's match after applying a set of operations.

With DeterministicOpSequence we are able to reproduce exactly the same
sequence of operations, over and over. However, if the filestore fails
(e.g., because we injected a failure), we want to check if it is kept
consistent after replaying its journal.

With VerifyFileStore, which extends DeterministicOpSequence, we are able
to bring a brand new filestore to the state the failed filestore would
reach had it not failed. We can then compare to check if the failure
introduced inconsistencies after replaying the journal.

(This is still work in progress and not fully functional)

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>