To make informed reclaim decisions, we need to have as much relevant
information as possible about our reclaim targets. Specifically, it
is useful to know how much memory is currently used by the JVM heap.

The reasoning behind this is that if pressure is coming from the heap,
ballooning will harm us, instead of helping us.

Note: This is really just a first approximation. Ideally, total memory
shouldn't matter, but rather memory delta since a last common event.
But counting memory is the initial first step for both.

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