brontide/conn: migrate to WriteMessage + Flush

This commit modifies WriteMessage to only perform encryption on the
passed plaintext, and buffer the ciphertext within the connection
object. We then modify internal uses of WriteMessage to follow with a
call to Flush, which actually writes the message to the wire.
Additionally, since WriteMessage does not actually perform the write
itself, the io.Writer argument is removed from the function signature
and all call sites.

brontide/conn.go

@@ -166,7 +166,11 @@ func (c *Conn) Write(b []byte) (n int, err error) {
 	// If the message doesn't require any chunking, then we can go ahead
 	// with a single write.
 	if len(b) <= math.MaxUint16 {
-		return len(b), c.noise.WriteMessage(c.conn, b)
+		err = c.noise.WriteMessage(b)
+		if err != nil {
+			return 0, err
+		}
+		return c.noise.Flush(c.conn)
 	}
 
 	// If we need to split the message into fragments, then we'll write
@@ -185,11 +189,15 @@ func (c *Conn) Write(b []byte) (n int, err error) {
 		// Slice off the next chunk to be written based on our running
 		// counter and next chunk size.
 		chunk := b[bytesWritten : bytesWritten+chunkSize]
-		if err := c.noise.WriteMessage(c.conn, chunk); err != nil {
+		if err := c.noise.WriteMessage(chunk); err != nil {
 			return bytesWritten, err
 		}
 
-		bytesWritten += len(chunk)
+		n, err := c.noise.Flush(c.conn)
+		bytesWritten += n
+		if err != nil {
+			return bytesWritten, err
+		}
 	}
 
 	return bytesWritten, nil