HA backfill issue when no lease exists for a time period

[cevian]

The code in HA that handles backfill has a subtle bug.

When no lease at all exists for a time period the code here sees that no lease exists for the replica in question and the time-period, and assumes that another replica has the lease.

But, in certain cases nobody has the lease for that time-period and that case isn't correctly handled (instead all replicas will drop the data). What should probably happen is that we create a lease for the "gap" period with the first replica that requests it.