os/FileStore.cc: insert not empty list<Context*> to op_finisher/ondisk_finisher.

Signed-off-by: Xinze Chi <xmdxcxz@gmail.com>
Merge pull request #3135 from XinzeChi/wip-3

os/FileStore.cc: insert not empty list<Context*> to op_finisher/ondisk_finisher

Reviewed-by: Sage Weil <sage@redhat.com>