mem: allow using io.WriterTo with a io.LimitedReader

The caller of this function can wrap the io.Reader in a
io.LimitedReader. This happens if some max message size is set. If so,
this `io.WriterTo` check doesn't work anymore. Work around this by
checking if it is maybe a `io.LimitedReader`.

Overall, the problem I'm trying to solve is that the constant

```go
		buf := pool.Get(readAllBufSize)
```

32KiB is way too much in our use case. Messages are typically at max
about only 1KiB in size so we always overallocate by ~31KiB in the best
case scenario so we want to use the `io.WriterTo` branch so that we
could appropriately size the buffer.

Signed-off-by: Giedrius Statkevičius <[email protected]>

Author: GiedriusS

mem/buffer_slice.go

@@ -257,6 +257,18 @@ func ReadAll(r io.Reader, pool BufferPool) (BufferSlice, error) {
 		_, err := wt.WriteTo(w)
 		return result, err
 	}
+
+	if lr, ok := r.(*io.LimitedReader); ok {
+		if wt, ok := lr.R.(io.WriterTo); ok {
+			// This is more optimal since wt knows the size of chunks it wants to
+			// write and, hence, we can allocate buffers of an optimal size to fit
+			// them. E.g. might be a single big chunk, and we wouldn't chop it
+			// into pieces.
+			w := NewWriter(&result, pool)
+			_, err := wt.WriteTo(w)
+			return result, err
+		}
+	}
 nextBuffer:
 	for {
 		buf := pool.Get(readAllBufSize)