Add peers returning recently requested blocks to sessions

[Stebalien]

Otherwise, we only add the first peer to return the block.

We may already do this, but I couldn't tell. Suggested by @olizilla.

[Stebalien]

(let's not put this into the mega patch, just keep it in mind for after we merge that)

[dirkmc]

On master, the keys for incoming blocks are passed to all sessions. The session checks if the keys are "interesting", meaning that the session has requested the corresponding blocks at some stage in the past (even if the session has since received the blocks). If the blocks are interesting the peer is added to the session.

In the PoC the SessionManager only notifies interested sessions of received blocks / HAVEs. The session adds the peers that sent those blocks. Session interest is cleared when the session ends.

[Stebalien]

Ah, I see. Thanks!