Add API to retrieve amount of pinned bytes on a node

[tmm360]

Summary

Is useful to know how much space is occupied on disk by pinned contents. Add an API to get these info from node. It should be easy with new localstorage implementation, I hope.

Motivation

Is a really useful information in context of node administration.

Implementation

Don't know about new localstorage implementation, but it should be easy to add a new API doing this.

Drawbacks

Don't see any.