This change is mostly mechanical, with the exception of the changes to the
`tower-batch` middleware. This middleware was adapted from `tower::buffer`,
and the `tower::buffer` code was changed to implement its own bounded queue,
because Tokio 0.3 removed the `mpsc::Sender::poll_send` method. See
ddc64e8d4d
for more context on the Tower changes. To match Tower as closely as possible
in order to be able to upstream `tower-batch`, those changes are copied from
`tower::Buffer` to `tower-batch`.
The name "Buffer" is changed to "Batch" everywhere, and the worker task is rewritten.
Instead of having Worker implement Future directly, we have a consuming async run() function.
There's a lot of functional overlap between the batch design and tower-buffer's
existing internals, so we'll just vendor its source code and modify it.
If/when we upstream it, we can deduplicate common components.
teor
Henry de Valence
Dimitris Apostolou