Encode the biggest allocated mirror ID in the last.dl.amnesia.boum.org CNAME record.

This feels like a more robust way to track what's the highest allocated mirror
ID than the previous loop. And contrary to the previous implementation, it
doesn't rely on the fact that there is no hole in the range of the NNN.dl.a.b.o
we've allocated (which should be more robust against mistakes in the pool
management).