Implement cache invalidation on the front-end

[jkoshy]

Once support for adding incremental changes to the data store is added (see #14, osmChange support to the dbmgr tool), we would need a way to invalidate 'stale' entries from the caches maintained inside the front-end servers.

A (too simple) solution could be to have the front-ends periodically reset their caches, for example by reading a 'generation count' of some kind from the data store at regular intervals and resetting their caches if this count has changed. Such an approach would however be wasteful because updates to the data store are likely to be frequent (once a minute with the Minutely Mapnik process), but with each update only touching a very small number of elements.