More specific wantlists

[hannahhoward]

Goals

Make handling of wantlists more clear, avoid memory copying with session tracking, remove unneeded mutexes

Implementation

Currently, the wantlist code obscures multiple different concerns:

• tracking of sessions (only needed for our own wantlists and our own outgoing wantlists for individual peers)
• thread safety
• bookkeeping fields only needed for incoming peer wantlists

All of this is made worse by the fact that every field on a wantlist entry becomes a field on a Bitswap message entry, even though they are unused

This PR takes the following approach:

• cut the wantlist public fields down to the ones that are used universally - CID & Priority
• handle thread safety outside the wantlist itself, since it's already largely handled that way
• move session tracking to private fields in a session tracked want list
• move the trash bookkeeping field into a private field only inside peer-request-queues

For Discussion

• Removing thread safety adds the most code changes to message queue. Could be a seperate PR
• fix Use different types for our wantlists and peer wantlists #13
• fix Concurrent map access in Entry.SesTrk #73

[Stebalien]

LGTM. I'm starting to think that the MessageQueue should be session-ignorant but I'm not sure how to do that and we can tackle it later.

[Stebalien]

At this point, we should be able to pass Entrys by value (saving an allocation and avoiding mutable shared state). However, we can fix that in a followup PR.

[Stebalien]

(fixed the rebase) (note to self: don't touch someone else's PR while they're working on it)

[Stebalien]

Extra field.