MINOR: htx: Add the function htx_change_blk_value_len()

MINOR: htx: Add the function htx_change_blk_value_len()

As its name suggest, this function change the value length of a block. But it
also update the HTX message accordingly. It simplifies the HTX API. The function
htx_set_blk_value_len() is still available and must be used with caution because
this one does not update the HTX message. It just updates the HTX block. It
should be considered as an internal function. When possible,
htx_change_blk_value_len() should be used instead.

This function is used to fix a bug affecting the 2.0. So, this patch must be
backported to 2.0.