Introduce extra indirection in tile paths.

This requires manual moving of tile directories. From data/tiles:

mkdir new
for i in `seq 0 99`; do mkdir new/$i; done
for i in `seq 1 20000`; do if test -d $i; then mv $i new/$(($i % 100))/; fi; done
mv new/* ./

(replace 20000 with an upper bound estimate on the number of scans in the system)