for readability

refac(bitswap:strategy) update interface

=)

the channel's "spin" is specified in the interface now =)

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