security: Support proxy with rate limiting and include CI test coverage for nginx rev proxy (#4373)

Previously Etherpad would not pass the correct client IP address through and this caused the rate limiter to limit users behind reverse proxies.  This change allows Etherpad to use a client IP passed from a reverse proxy.

Note to devs: This header can be spoofed and spoofing the header could be used in an attack.  To mitigate additional *steps should be taken by Etherpad site admins IE doing rate limiting at proxy.*  This only really applies to large scale deployments but it's worth noting.

tests/ratelimit/send_changesets.js

@@ -0,0 +1,25 @@
+try{
+  var etherpad = require("../../src/node_modules/etherpad-cli-client");
+  //ugly
+} catch {
+  var etherpad = require("etherpad-cli-client")
+}
+var pad = etherpad.connect(process.argv[2]);
+pad.on("connected", function(){
+
+  setTimeout(function(){
+    setInterval(function(){
+      pad.append("1");
+    }, process.argv[3]);
+  },500); // wait because CLIENT_READY message is included in ratelimit
+
+  setTimeout(function(){
+    process.exit(0);
+  },11000)
+});
+// in case of disconnect exit code 1
+pad.on("message", function(message){
+  if(message.disconnect == 'rateLimited'){
+    process.exit(1);
+  }
+})