BUG/MINOR: stconn: Use HTX-aware channel's functions to get info on buffer

BUG/MINOR: stconn: Use HTX-aware channel's functions to get info on buffer

Some channel function are used to check if the channel's buffer is full, not
empty or if there are input data. However, functions used are not
HTX-aware. So it is not accurate and may prevent some actions to be
performed (However, not sure there are really issues). Because HTX-aware
versions now exist, use them instead.

This patch may be backported as far as 2.2. It relies on

    * "MINOR: channel: Add functions to get info on buffers and deal with HTX streams"
    * "MINOR: htx: Use a macro for overhead induced by HTX"