Split metadata out of PooledRef

[simonbasle]

PooledRef should only directly expose the methods that are most relevant to the majority of users: release(), invalidate() (to a lesser extent, but that's the logical place to put it) and poolable().

Additional methods that return metrics about the PooledRef should be hidden away in a separate interface. Since these can be viewed as metadata for the reference (which could grow beyond simple metrics to include some state), I suggest PooledRefMetadata.

The metadata would be accessible through a fourth method, PooledRef#metadata().

In order to limit garbage, internal concrete implementations of PooledRef should directly implement PooledRefMetadata and have metadata() return a view of this.

[simonbasle]

Originally discussed in #4 (comment),
#4 (comment) and #4 (comment)