veyron/tools/playground: Add response caching to the playground.
veyron/tools/playground: Add response caching to the playground.

Uses an LRU cache (golang/groupcache/lru). Dependent on this CL:
https://veyron-review.googlesource.com/#/c/4253/

Also implements a max size of 65k for both the program and response
size.

Change-Id: Ifabb12fdda543d372377bcda9f6995c02e85fe2f

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref