Reduce bandwidth at low zoom level: 'trick' the map into showing lower zoom levels.

For example, if the map is at zoom level x, show the tiles for x - 4. I don't do this exactly.

This code has a low/high zoom level switch, at
the high zoom level, behave as before this commit; at the low zoom
level, trick the map into showing very low resolution tiles, so only a
few tiles need loading to cover the map area.
Rename to HIGH_ZOOM_THRESHOLD to be consistent with https://github.com/mozilla/MozStumbler/pull/998/files#diff-ecc6477a0a55adf1f37c8d8de23f5f85R284