Change tile layer url doesn't always discard all old tiles

[manthey]

Occasionally, it looks like a tile still uses the old url. This is infrequent, so perhaps it is only when the tile has a pending callback or something similar where it can occur. The behavior can sometimes be reproduced when doing A/B level swapping often. In those cases, it also looks like some of the tiles which should be lazy (off screen) are still being counted toward idleness.

[manthey]

Digging in to this, it looks like the python use of memcache sometimes leaks old data and has nothing to do with geojs.