os/bluestore: add flush_store_cache cmd

os/bluestore: add flush_store_cache cmd

We want this cmd for bluestore testing, e.g., to exclude the impact of
the bluestore internal cache without rebooting host.

e.g.:
[root@tecs131 ~]# ceph --admin-daemon /run/ceph/ceph-osd.1.asok perf dump | grep "bluestore_buffers"
        "bluestore_buffers": 3166,
[root@tecs131 ~]# ceph --admin-daemon /run/ceph/ceph-osd.1.asok flush_store_cache
[root@tecs131 ~]# ceph --admin-daemon /run/ceph/ceph-osd.1.asok perf dump | grep "bluestore_buffers"
        "bluestore_buffers": 95,

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>