use store_opts to vehicle offset and load/store data size for disk operations

[tomcucinotta]

The store_opts (currently unused) structure might be useful to emulate sequential vs random data access patterns.
Also, it might be useful to tell the server what storage device to use, e.g., though a simple dev_id identifier (0, 1, 2, ...), to exploit multiple disk devices if available on the server, for emulating disk-related workloads taking advantage of multiple disks (e.g., data stores).

Related issue: #20