BUG/MEDIUM: lua: reset lua transaction between http requests

Previously LUA code would maintain the transaction state between http
requests, resulting in things like txn:get_priv() retrieving data from
a previous request. This addresses the issue by ensuring the LUA state
is reset between requests.

Co-authored-by: Tim D�sterhus <tim@bastelstu.be>

reg-tests/lua/h00001.lua

@@ -0,0 +1,15 @@
+core.register_action("bug", { "http-res" }, function(txn)
+	data = txn:get_priv()
+	if not data then
+		data = 0
+	end
+	data = data + 1
+	print(string.format("set to %d", data))
+	txn.http:res_set_status(200 + data)
+	txn:set_priv(data)
+end)
+
+core.register_service("fakeserv", "http", function(applet)
+	applet:set_status(200)
+	applet:start_response()
+end)

reg-tests/lua/h00001.vtc

@@ -0,0 +1,31 @@
+varnishtest "Lua: txn:get_priv() scope"
+feature ignore_unknown_macro
+
+haproxy h1 -conf {
+    global
+        lua-load ${testdir}/h00001.lua
+
+    frontend fe1
+        mode http
+        bind "fd@${fe1}"
+        default_backend b1
+
+        http-response lua.bug
+
+    backend b1
+        mode http
+        http-request use-service lua.fakeserv
+} -start
+
+client c0 -connect ${h1_fe1_sock} {
+    txreq -url "/"
+    rxresp
+    expect resp.status == 201
+    txreq -url "/"
+    rxresp
+    expect resp.status == 201
+}
+
+client c0 -start
+
+client c0 -wait