Split barrier operations that contain data

[ashmrtn]

Flush operations are defined oddly. They make sure that the data in the device cache is persisted, but will not make sure the data in the request itself is persisted (link). In light of this, CrashMonkey should split flush operations that have data. The flush operation itself should end the current epoch in CrashMonkey, but the data should be placed in the next epoch as it is not guaranteed to be persisted in the epoch the flush just ended. This should probably be done in the Permuter class when it initializes internal data structures so that it this behavior is transparent to user implemented permuters.