speed up incremental builds by not doing excessive stats.toJSON work (#1362)

* speed up incremental builds by not doing excessive stats.toJSON work

* turn more fields off from stats.toJSON since those other fields are not used

Author: kenotron

lib/Server.js

@@ -25,7 +25,7 @@ const createLog = require('./createLog');
 const OptionsValidationError = require('./OptionsValidationError');
 const optionsSchema = require('./optionsSchema.json');
 
-const clientStats = { errorDetails: false };
+const clientStats = { all: false, assets: true, warnings: true, errors: true, errorDetails: false };
 
 function Server(compiler, options, _log) {
   // Default options