Avoid keeping blocks in memory longer than we need to

[Stebalien]

When we get a block, we currently send it to every interested session (SessionManager.ReceiveBlocksFrom). Looking into this, we mostly just need the CID. That would:

1. Save us some memory (sometimes).
2. Avoid keeping blocks we didn't ask for around. We currently do this to track duplicates but I'd rather toss them ASAP.

We still need the blocks themselves to send them to the user but we should be able to avoid that by removing the per-session PubSub instance and reverting to a global PubSub instance.

[dirkmc]

I agree, it doesn't seem like the Session needs to know about PubSub or receive the blocks themselves (just the block CIDs). As you say we should be able to handle this with a global PubSub instance, and it would simplify the code.