test: test_store_tool: add 'crc <prefix> <key>' command

Returns the CRC of contents for a given key with a given prefix.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
test: test_store_tool: optionally output value crc when listing keys

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
ceph_test_store_tool: add 'set prefix key' feature

Allow reading from a file.  See --help for more info.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #677 from ceph/wip-store-tool

wip-store-tool: Few patches to ceph_test_store_tool

Reviewed-by: Sage Weil <sage@inktank.com>