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.
2 files changed