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Allows the service to be used through an interface.

NB: If the handler is exposed directly, clients of the service cannot swap out their concrete references and replace them with interfaces

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

We were issuing handling goroutines in both NewService and Start

@perfmode this means we can create a Service first, give it to
the network and protocol (removing interdep).

fix(net:msg) use vendored imports