Doing
```rust
let clone = self.inner.clone();
Box::pin(async move {
let response = clone.call(request).await?;
Ok(response)
})
```
If `self.inner` is (or contains) a `tower::buffer::Buffer` might panic.
That is because cloning a `Buffer` drops the permit that was acquired in
`poll_ready`, meaning it is no longer ready and panic in `call`.
The solution is to use `mem::replace` to take the ready service and pass
that into the async block.
Fixes https://github.com/hyperium/tonic/issues/545
