[CTX] [ALL] header filtering

kill-[i][qr]head: listOfHeaders
erase header fields listed in listOfHeaders before forwarding a request/response message to server/client. "kill-qhead" is applied only to request message to server and "kill-rhead" is applied only to response message to client. If "i" is prefixed as "kill-iqhead:Pragma,Cache-Control", the specified fields are erased before DeleGate interpret it as the HTTP headers.