`v.([]byte)` coming from a datastore can panic.
`byt, ok := v.([]byte)` to be safe.

@whyrusleeping

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

makes more sense this way

remove this setter while the interface is under construction

it'll have to change lots since the listener is gone

@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).

* to wrap datastore for ease of use
* to pass a non-responsive bitswap mock rather than performing nil
* checks internally

no longer exists. instead, use net message