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
