Store a complete wantlist

[Stebalien]

As far as I can tell, sessions mean that we don't actually store a complete wantlist anywhere. This is extremely frustrating from a UX standpoint and makes solving #21 difficult.

[hannahhoward]

@Stebalien

As far as I can tell, the in WantManager struct, the wl wantlist.Threadsafe field contains a complete list of wanted block CIDs, even with sessions.

Am I missing something? Here's how I get there:

1. sessions.go and bitswap.go both call WantBlocks on the WantManager
2. want manager calls internal method addEntries which sends a message to an internal incoming channel that gets picked up by Run function. The run function adds all incoming blocks to the wl wantlist.Threadsafe queue (I was initially confused by the distinction between bcwl & wl but I now see while sessions will prevent blocks from ending up on bcwl, all blocks wanted end up on wl based on the Run code.
3. The internal implementation of wantlist.ThreadSafe's AddEntry method should prevent duplicate blocks from appearing on the wantlist with sessions, but still track all sessions a block is needed for.

All of this is somewhat supported by the fact that Bitswap's GetWantList method calls b.wm.wl.Entries()

Anyway, I could be super wrong -- I just want to understand what I'm missing so I know what to fix.

[Stebalien]

No, that sounds right. That makes this much simpler.