tools: ceph-monstore-tool: allow copying store contents to a new location

Usage:

ceph-monstore-tool --mon-store-path <in-path> --out <out-path> \
                   --command store-copy

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #496 from ceph/wip-monstore-copy

tools: ceph-monstore-tool: copy a store's contents to another store