Improve Pin-Marking Speed in Dense Areas

[milo-trujillo]

We've been getting reports that marking a camera now takes over 30 seconds for some users in particularly camera-rich regions. This is unacceptable both in terms of visual feedback (see #46), but also in terms of server performance. Right now we compare the new potential camera to each camera in its zone-file, to check for overlaps with an existing camera. That works fine with a zone-file of a hundred or less cameras, but may be extremely inefficient for a densely packed area.

We need to improve our database schema and querying mechanism to get that performance way down from O(n). It was a shoddy db design to begin with, and now that corner-cutting has caught up with us.

[milo-trujillo]

There is a small chance that this was not tied to the camera database design (#56), but was instead tied to the login database design (#57). If this is the case, then new users should be unaffected by this issue, and existing users will be unaffected after marking a single camera (which will migrate them to the new login database).

We need to do some testing to confirm whether this issue still exists.