libmem: implement policy-agnostic memory allocation/accounting.

[klihub]

Note: This PR has been rebased on #358, which should be should reviewed first.

This patch series

• implements a policy-agnostic library for memory allocation/accounting
• replaces existing memory accounting in the topology-aware policy with libmem
• plugs in libmem-based memory accounting and allocation to the balloons policy

For a more detailed description of the main ideas, core concepts, and some high level
implementation details, see the included package documentation.

TODO items:

• get topology-aware: don't ignore HBM memory nodes without close CPUs. #329 merged then rebase
• add missing ensureNormalMemory() during initial zone selection
• allow for both strict and preferred memory type selection
• add an interface for overriding builtin zone expansion and overflow resolution
• make sure memory type preference is consistently either a preference or a requirement
• add a description of how the allocator works/describe the basic idea
• comment/document zone expansion, zone overflow resolution
• clean up debug-logging to be actually helpful
• replace inertia with a more granular priority, with pre-defined priorities for QoS classes

Remaining questions related to this initial implementation:

• Are the builtin defaults good enough ?

[askervin]

....starting review. I could not yet follow through the whole path if it is or not possible that PreserveContainer requests can return zones different from originals. But if they do... for instance if a single zone of size 32 GB is the only zone that is allowed for two preserved-memory containers, each requesting 32 GB of RAM, then I'd still assume that whoever did this preservation must know what he is doing. If he prefers oom kill of his containers rather than letting them access memory out of the zone, then so be it, we won't touch pinning.