The COPY OUT protocol allows sending CopyData packets that have no data.
The (synchronous) CopyOutReader needs to be careful not to return an
empty slice in this case, but instead request more data, since an empty
slice is taken to mean EOF.
This avoids a bunch of context switches and cross-thread
synchronization, which ends up improving the performance of a simple
query by ~20%, from 252us to 216us.
Rather than taking in a Stream and advancing it internally, return a
Sink that can be advanced by the calling code. This significantly
simplifies encoding logic for things like tokio-postgres-binary-copy.
Similarly, the blocking interface returns a Writer.
Closes#489
