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Before, the engine worker would pop a task and block on send to the
bitswap worker even if the bitswap worker wasn't to receive. Since the
task could have been invalidated during this blocking send, a small
number of stale (already acquired) blocks would be send to partners.

Now, tasks are only popped off of the queue when bitswap is ready to
send them over the wire. This is accomplished by removing the
outboxChanBuffer and implementing a two-phase communication sequence.