This commit refactors the middleware application logic in the webpack-dev-server codebase to ensure that middleware is applied only once, regardless of how many times it's called. 

Previously, certain middleware, such as static serving middleware, was being applied multiple times to support features like `historyApiFallback`. This refactor eliminates duplicate middleware application by introducing a mechanism to track applied middleware and applying each middleware only once.

Changes:
- Introduced `appliedMiddleware` array to track applied middleware.
- Created `applyMiddlewareOnce` function to apply middleware only if it hasn't been applied before.
- Updated webpack-dev-server codebase to utilize `applyMiddlewareOnce` for applying middleware associated with various features.

This update aims to improve code efficiency and prevent unintended behavior or performance issues caused by duplicate middleware application.

Fixes: webpack#2716

In this updated version, I have included test cases to ensure that the applyMiddlewareOnce function behaves as expected. Additionally, I have removed references to contentBase, as requested.