Additional in-process cache plugin InProcessCache

Additional in-process cache plugin InProcessCache

Some of our caching systems, like the disk cache or memcached, have
significant overhead (network connections or disk I/O).

This plugin adds an additional layer of in-process cache, so we don't
need to reconnect to external cache systems when we've already
received a data item from the cache. There are some concurrency issues
here, but typically they won't be important at the level of a single
web hit.