mempool: memory statistics

This patch adds the basic of memory tracking, and exposes an interface to for
that data to be collected.

We basically start with all stats at zero, and as we add memory to the System,
we bump it up and recalculate the watermarks (to avoid recomputing them all the
time). When a page range comes up, it will be added as free memory.

We operate based on what is currently sitting in the page ranges. This means
that we are effectively ignoring memory that sit in pools for memory usage. I
think it is a good assumption because it allow us to focus in the big picture,
and leave the pools to be used as liquid currency.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>