MINOR: conn_stream: add an tx buffer to the conn_stream

To be symmetrical with the recv() part, we no handle retryable and partial
transmission using a intermediary buffer in the conn_stream. For now it's only
set to BUF_NULL and never allocated nor used.

It cannot yet be used as-is without risking to lose data on close since
conn_streams need to be orphaned for this.
diff --git a/include/types/connection.h b/include/types/connection.h
index 83c6bd1..cf5dbb6 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -373,6 +373,7 @@
 	struct wait_list wait_list;          /* We're in a wait list for send */
 	struct list send_wait_list;          /* list of tasks to wake when we're ready to send */
 	struct buffer rxbuf;                 /* receive buffer, always valid (buf_empty or real buffer) */
+	struct buffer txbuf;                 /* transmission buffer, always valid (buf_empty or real buffer) */
 	void *data;                          /* pointer to upper layer's entity (eg: stream interface) */
 	const struct data_cb *data_cb;       /* data layer callbacks. Must be set before xprt->init() */
 	void *ctx;                           /* mux-specific context */