Send retry has been removed from the multiSender. This also means there is not need for the active func, because we simply wait until the send is complete or failed to exit the output clips
routine. Tests pertinent to retrying or the active function have been removed.
Added test to check that if a sender is set to retry on send fails,
it will keep trying to send until it is successful, and have all
data that was intended to be written sent off.
Added a test in senders_test.go to check that we can give a multiSender a few senders and have it
write to them correctly when we call multSender.Write(). This involved writing a dummy
loadSender implementation called dummyLoadSender that stores what has been written to it so that
we can check at a later time.
Saxon