MEDIUM: lua: Improve/revisit the lua api to manipulate channels

MEDIUM: lua: Improve/revisit the lua api to manipulate channels

First of all, following functions are now considered deprecated:

  * Channel:dup()
  * Channel:get()
  * Channel:getline()
  * Channel:get_in_len()
  * Cahnnel:get_out_len()

It is just informative, there is no warning and functions may still be
used. Howver it is recommended to use new functions. New functions are more
flexible and use a better naming pattern. In addition, the same names will
be used in the http_msg class to manipulate http messages from lua filters.

The new API is:

  * Channel:data()
  * Channel:line()
  * Channel:append()
  * Channel:prepend()
  * Channel:insert()
  * Channel:remove()
  * Channel:set()
  * Channel:input()
  * Channel:output()
  * Channel:send()
  * Channel:forward()
  * Channel:is_resp()
  * Channel:is_full()
  * Channel:may_recv()

The lua documentation was updated accordingly.