BUG/MEDIUM: hlua: Fix end of request detection when retrieving payload

BUG/MEDIUM: hlua: Fix end of request detection when retrieving payload

When the lua HTTP applet was refactored to use its own buffers, a bug was
introduced in receive() and getline() function. We rely on HTX_FL_EOM flag
to detect the end of the request. But there is nothing preventing extra
calls to these function, when the whole request was consumed. If this
happens, the call will yield waiting for more data with no way to stop it.

To fix the issue, APPLET_REQ_RECV flag was added to know the whole request
was received.

This patch should fix #3293. It must be backported to 3.3.