NewManager creates a new Manager.

DefaultPolicy randomly selects pieces to request.

RarestFirstPolicy selects pieces that the fewest of our peers have to request first.

StatusExpired denotes an in-flight request which has timed out on our end.

StatusInvalid denotes a completed request that resulted in an invalid payload.

StatusPending denotes a valid request which is still in-flight.

StatusUnsent denotes an unsent request that is safe to retry to the same peer.

Manager encapsulates thread-safe piece request bookkeeping.

Request represents a piece request to peer.

Status enumerates possible statuses of a Request.