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When sending data to another user, the number of bytes sent to that user (saved
by the corresponding Bitswap ledger) was not updated (it was always 0). This
also meant that the debt ratio was also always 0.

The function that updates the `BytesSent` value in the ledger, `MessageSent()`,
was already implemented, however it was not called when the peer was sent data.
To fix this, a call to `MessageSent()` was made in the `taskWorker()` function,
which is where both the message in question and the Bitswap engine were
available to make the call. `MessageSent()` requires the peer's ID and
`BitSwapMessage` as its arguments, the latter of which had to be created by
making a new `BitSwapMessage`, then the block being sent was added to the new
message.

Note that, similar to the analagous call to `MessageReceived()`, records *all*
of the bytes sent to a particular user. At some point, both of these should be
updated to only record the numbers of *useful* bytes sent and received between
peers.

License: MIT
Signed-off-by: David Grisham <dgrisham@mines.edu>