MINOR: httpclient/lua: handle the streaming into the lua applet

MINOR: httpclient/lua: handle the streaming into the lua applet

With this feature the lua implementation of the httpclient is now able
to stream a payload larger than an haproxy buffer.

The hlua_httpclient_send() function is now split into:

hlua_httpclient_send() which initiate the httpclient and parse the lua
parameters

hlua_httpclient_snd_yield() which will send the request and be called
again to stream the request if the body is larger than an haproxy buffer

hlua_httpclient_rcv_yield() which will receive the response and store it
in the lua buffer.