revid: replace method value approach used for clip sending

The current approach to managing clip sending work is via a method value closure. This makes it difficult to separate clip loading to the worker from clip sending by the worker. This raises a problem for the case where we are sending via HTTP and need to repeatedly copy the ring chunk into a buffer to be thrown away even though it may be used again for a retry.

I would like to change the type of the worker from a func(*ring.Chunk) error to an interface.

type loadSender interface {
    load(*ring.Chunk) error
    send() error
}

This has the additional advantage that we can then instead of keeping a loadSender field in revid, we can have a []loadSender with an arbitrary set of destination protocols.

Note that the load is quick (either constant time or linear in the length of the chunk for the case of HTTP), but the send can be slow due to network; the send operations can be wrapped in a goroutine for each protocol so that they can be performed concurrently.

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