Factor out cell validation in search_all_sources.
Rewrite search_cell into using a single DB query for multiple cells at once.
Rewrite search_cell_lac into using a single DB query.

And only search for each lac once, even if we get multiple cells
from the same lac.
Avoid ORM overhead for api key query.
Avoid assert statements in non-test code.
flake8
Let `insert_*_measures` tasks expire after two hours to avoid queue overload.

If the system cannot process incoming data fast enough, drop the data on
the floor instead of breaking the system.