Minor performance improvements (facebook#7587)

- use ESLint cache
- remove useless `watch` option in fork-ts-checker

Author: deftomat

packages/react-scripts/config/webpack.config.js

@@ -334,6 +334,7 @@ module.exports = function(webpackEnv) {
           use: [
             {
               options: {
+                cache: true,
                 formatter: require.resolve('react-dev-utils/eslintFormatter'),
                 eslintPath: require.resolve('eslint'),
                 resolvePluginsRelativeTo: __dirname,
@@ -690,7 +691,6 @@ module.exports = function(webpackEnv) {
             '!**/src/setupProxy.*',
             '!**/src/setupTests.*',
           ],
-          watch: paths.appSrc,
           silent: true,
           // The formatter is invoked directly in WebpackDevServerUtils during development
           formatter: isEnvProduction ? typescriptFormatter : undefined,