dm cache: tune migration throttling

Tune the dm cache migration throttling.

i) Issue a tick every second, just in case there's no i/o going through.

ii) Drop the migration threshold right down to something suitable for
background work.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>