Enable custom task prioritization logic

[synzhu]

This is the second part of ipfs/boxo#82.

It introduces a TaskComparator which can be configured to implement custom task prioritization logic.

[aschmahmann]

Thanks :). Can you add a test showing that this works? It can also serve as a useful example for people who want to leverage this option.

[synzhu]

Thanks :). Can you add a test showing that this works? It can also serve as a useful example for people who want to leverage this option.

@aschmahmann Sure, I can try, I thought about it but originally I didn't see any way to do black box testing of this functionality since it would have to somehow rely on timing. But I can probably do some sort of whitebox test that asserts on the order of PeerTasks popped from the PeerTaskQueue

[Stebalien]

Small nits about type accessibility, but othewise LGTM. In the future, we'll probably want to expose a peer comparator directly, but that shouldn't be a breakign change.

[Stebalien]

Thanks!