dm-snap-persistent: make metadata_wq multithreaded

metadata_wq serves on-stack work items from chunk_io().  Even if
multiple chunk_io() are simultaneously in progress, each is
independent and queued only once, so multithreaded workqueue can be
safely used.

Switch metadata_wq to multithread and flush the work item instead of
the workqueue in chunk_io().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: dm-devel@redhat.com