create API to communicate checkpoint requests from CrashMonkey test harness to disk_wrapper module

[ashmrtn]

Part of the revised version of #12.

Checkpoints require support across many parts of CrashMonkey. This part is meant to provide the ability for the CrashMonkey test harness to tell the disk_wrapper to create a checkpoint. This can be implemented as an ioctl call to the disk_wrapper.

For checkpoints, we can assume 2 things:

1. the user has just performed a sync/fsync request of some form
2. this call will block until all parts of the checkpoint are completed

The ioctl should be a synchronous call. The work done in the ioctl is in #39.