temporarily. at least until refactor is complete

temporarily. until bitswap is refactored

will have persistence later on as a datastore passed in.

this will later have persistent storage, but no need yet

`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