Expose stats for prepared statement cache

[plaflamme]

Prepared statements are cached on a per-connection basis. This cache has a maximum number of elements and will evict entries when necessary. This can lead to runtime performance issues and so needs to be observable.

finagle-mysql does the same and exposes stats for its cache. The same stats should be exposed by this client.

The task is to implement the TODO here following the same pattern present in finagle-mysql here.