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makes more sense this way

remove this setter while the interface is under construction

@perfmode sender is exactly what we need to pass in to dht/bitswap.

We were issuing handling goroutines in both NewService and Start

no longer exists. instead, use net message

discard erroneous values

wherever blocks cannot be nil, use value rather than pointer. only use
pointers when absolutely necessary.

rather than pointer

add interface

use a third-party pubsub library for internal communications

Insights:
* Within bitswap, the actors don't need anything more than simple pubsub
behavior. Wrapping and unwrapping messages proves unneccessary.

Changes:
* Simplifies the interface for both actors calling GetBlock and actors
receiving blocks on the network
* Leverages a well-tested third-party pubsub library

Design Goals:
* reduce complexity
* extract implementation details (wrapping and unwrapping data, etc)
from bitswap and let bitswap focus on composition of core algorithms
operations