Move rx_post_buffers to PipeConnection, make introspectable.

This removes rx_buffers logic from the base Connection class, and
makes explicit when upper-layer code can rely on it.

On second look, the logic in rgw/rgw_rest_s3.h is using an unrelated
"rx_buffer" and should not be affected.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>