dog/farm: add an option for multi-thread cluster snapshot save process

There is no reason we can't multi-thread it. Let's go multi-threading and enjoy
boost.

Consider that we often save snapshots on the fly so we use single thread as
default to minimize the impact of running cluster.

usage:
 $ dog cluster snapshot save -m ... # enable multi-threads

[Hitoshi: added "m" to the accepted options of cluster snapshot
command]

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>