Latency awareness

[wprzytula]

This is the first attempt to introduce latency awareness. I've chosen to build it atop the current DefaultPolicy, because it is natural that LA simply adds yet another level of iterator permutation.

The PR includes identification-by-HostID PR. This is for identifying nodes in LatencyAwareness' HashMap.

New things are in last 4 commits.

[wprzytula]

@havaker @cvybhu Waiting for opinions on design adopted.

[wprzytula]

Addressed both comments:

• Box'ed predicate instead of a generic
• is_enabled() stub for is_alive()

[havaker]

Please drop this commit a114513

[wprzytula]

Please drop this commit a114513

Ok, but now latency_aware.rs is not deleted; I've made a mistake during recent rebase on main, so we probably need to remove it again. I would suggest removing it in a new commit, and I'll handle it in a final cleanup of this PR.

[havaker]

Please rebase to havaker:lb2.

[wprzytula]

Please rebase to havaker:lb2.

done.