Forward framed read buf contents to compute before proxy pass.

Otherwise they get lost. Normally buffer is empty before proxy pass, but this is
not the case with pipeline mode of out npm driver; fixes connection hangup
introduced by b80fe41af3 for it.

fixes https://github.com/neondatabase/neon/issues/3822

proxy/src/stream.rs

@@ -1,5 +1,6 @@
 use crate::error::UserFacingError;
 use anyhow::bail;
+use bytes::BytesMut;
 use pin_project_lite::pin_project;
 use pq_proto::framed::{ConnectionError, Framed};
 use pq_proto::{BeMessage, FeMessage, FeStartupPacket, ProtocolError};
@@ -27,8 +28,8 @@ impl<S> PqStream<S> {
         }
     }
 
-    /// Extract the underlying stream.
-    pub fn into_inner(self) -> S {
+    /// Extract the underlying stream and read buffer.
+    pub fn into_inner(self) -> (S, BytesMut) {
         self.framed.into_inner()
     }