clean up of cell_backfill population code to remove table scans
https://github.com/mozilla/ichnaea/pull/69#discussion_r8699178
Filter out records where MCC and MNC are 0 during the initial
population of the cell_backfill table.

https://github.com/mozilla/ichnaea/pull/69#discussion_r8699239
switch the CellBackfill table to use a MEMORY storage backend in mysql
Clean up of the cell_backfill table added after backfill task has completed