Command to report %-in memory per namespace

Add a command which can report the % of the namespace (for both the data and all indexes related to a collection) which is currently in (mapped-resident/cache) memory. This can be added for reporting cache/memory efficiency and for debugging performance issues.

It could be called something like

{cacheStats: <ns>}

and report something like this:
{cacheStats: [
{<ns>:

{ percent: 20, graph: "XXXXXXX___0000_____XXXXXX..." ]}

Where percent is the percent in memory of the namespace, and a graphic (textual) representation shows which parts of the namespace are in memory.

Or a distribution is the number of the contiguous page blocks (which could tell you if you are accessing data randomly or in ranges, and should be cheap to calculate as you are walking pages ~ mincore).
distribution: [
4: 20000
25: 60
300: 4
...
]}}