ceph-kvstore-tool: calc store crc

Reuse 'list()' function to traverse the store and calc not only version's
crcs, but also calc the store's crc.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
ceph-kvstore-tool: copy one leveldb store to some other place

Iterates over the provided source store's keys and copies them to the
provided destination store.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>