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

fix(net:msg) use vendored imports