BUG/MINOR: proto_htx: Use HTX versions to truncate or erase a buffer

BUG/MINOR: proto_htx: Use HTX versions to truncate or erase a buffer

channel_truncate() is not aware of the underlying format of the messages. So if
there are some outgoing data in the channel when called, it does some unexpected
operations on the channel's buffer. So the HTX version, channel_htx_truncate(),
must be used. The same is true for channel_erase(). It resets the buffer but not
the HTX message. So channel_htx_erase() must be used instead. This patch is
flagged as a bug, but as far as we know, it was never hitted.

This patch should be backported to 1.9. If so, following patch must be
backported too:

  * MINOR: channel/htx: Add the HTX version of channel_truncate/erase