http: reuse socket only when it is drained

Preserve 'prefinish' event

lib/_http_client.js

@@ -351,6 +351,7 @@ ObjectSetPrototypeOf(ClientRequest.prototype, OutgoingMessage.prototype);
 ObjectSetPrototypeOf(ClientRequest, OutgoingMessage);
 
 ClientRequest.prototype._finish = function _finish() {
+  FunctionPrototypeCall(OutgoingMessage.prototype._finish, this);
   if (hasObserver('http')) {
     startPerf(this, kClientRequestStatistics, {
       type: 'http',

lib/_http_outgoing.js

@@ -40,6 +40,7 @@ const {
 } = primordials;
 
 const { getDefaultHighWaterMark } = require('internal/streams/state');
+const assert = require('internal/assert');
 const EE = require('events');
 const Stream = require('stream');
 const internalUtil = require('internal/util');
@@ -984,11 +985,12 @@ OutgoingMessage.prototype.end = function end(chunk, encoding, callback) {
 };
 
 
-// Subclasses HttpClientRequest and HttpServerResponse should
-// implement this function. This function is called once all user data
-// are flushed to the socket. Note that it has a chance that the socket
-// is not drained.
-OutgoingMessage.prototype._finish = function() {};
+// This function is called once all user data are flushed to the socket.
+// Note that it has a chance that the socket is not drained.
+OutgoingMessage.prototype._finish = function _finish() {
+  assert(this.socket);
+  this.emit('prefinish');
+};
 
 
 // This logic is probably a bit confusing. Let me explain a bit:

lib/_http_server.js

@@ -222,6 +222,7 @@ ServerResponse.prototype._finish = function _finish() {
       },
     });
   }
+  OutgoingMessage.prototype._finish.call(this);
 };