http: add maxTotalSockets to agent class

doc/api/http.md

@@ -308,7 +308,7 @@ added: REPLACEME
 * {number}
 
 By default set to `Infinity`. Determines how many concurrent sockets the agent
-can have open.
+can have open. Unlike `maxSockets`, this parameter across all origins.
 
 ### `agent.requests`
 <!-- YAML

lib/_http_agent.js

@@ -44,6 +44,7 @@ const { once } = require('internal/util');
 const { validateNumber } = require('internal/validators');
 
 const kOnKeylog = Symbol('onkeylog');
+const kRequestOptions = Symbol('requestOptions');
 // New Agent code.
 
 // The largest departure from the previous implementation is that
@@ -284,7 +285,7 @@ Agent.prototype.addRequest = function addRequest(req, options, port/* legacy */,
     }
 
     // Used to create sockets for pending requests from different origin
-    req._options = options;
+    req[kRequestOptions] = options;
 
     this.requests[name].push(req);
   }
@@ -428,17 +429,20 @@ Agent.prototype.removeSocket = function removeSocket(s, options) {
     debug('removeSocket, have a request, make a socket');
     req = this.requests[name][0];
   } else {
+    // TODO(rickyes): this logic will not be FIFO across origins. There might be 
+    // older requests in a different origin, but if the origin which releases
+    // the socket has pending requests that will be prioritized.
     for (const prop in this.requests) {
       debug('removeSocket, have a request with different origin,' +
         ' make a socket');
       req = this.requests[prop][0];
-      options = req._options;
+      options = req[kRequestOptions];
       break;
     }
   }
 
   if (req && options) {
-    delete req._options;
+    delete req[kRequestOptions];
     // If we have pending requests and a socket gets closed make a new one
     this.createSocket(req, options, (err, socket) => {
       if (err)