MINOR: lua: replace function (req|get)_channel by a variable

MINOR: lua: replace function (req|get)_channel by a variable

To add data in channel, it is necessary to process in two times.
First time, get the channel object, and after send data:

   local req = txn:req_channel()
	req:send("data\n")

Now, the function is converted as a variable containing the req
and res aobject. We can process as following:

   txn.req:send("data\n")