PG: switch the start_recovery_ops interface to specify work to do as a param

PG: switch the start_recovery_ops interface to specify work to do as a param

We previously inferred whether there was useful work to be done
by looking at the number of ops started, but with the upcoming
introduction of the rw_manager read locking on backfill, we could
start no ops while still having work to do. Switch around the
interfaces to specify these as separate pieces of information.

Signed-off-by: Greg Farnum <greg@inktank.com>