Fix for VNX-27, VNX-28, VXN-33

Initial commit for sync-snap support (VNX-27). Changes to LogicalStorageManager interface.

Supports explicit snapPoint creation and finishing of snapshots as two distinct phases.
DASManager and OverlayManager implement the interface but perform no-ops and behave as before.

BlockStorage always calls createSnapshotPoint() synchronously on the createSnapshot() call from
the CLC. If the return value from createSnapshotPoint() is null then the operation will be done
completely asynchronously as before, but if not then before returning to the CLC the SC has set
the snap consistency point on the backend and will continue the rest of the snapshot asynchronously.

This allows the user to be confident that by the time the euca-create-snapshot call has returned
the snap consistency point has been set and all subsequent I/Os to the volume will not be included
in the snapshot.