Better metrics for in-memory caches

[bhearsum]

Balrog has a number of in-memory caches for, primarily, results from the database and autograph. At the moment, we have zero insight into how well these perform: the hit rates, the refresh rates, whether or not we dump things out to make room for new things. It would be wonderful to have this information produced in a way that we can graph in graphana (or somewhere else).

@jcristau - does this cover what you were thinking of?

[jbuck]

as a first pass at this, could you output some statsd metrics on cache requests, hits, and misses?