🏗 Re-implement gulp serve as an in-process server

[rsimha]

PR Highlights:

• Removes the external-process implementation based on nodemon + gulp-webserver (hasn't been updated in 4 years)
• Adds an in-process implementation based on gulp-connect (updated more recently)
• Removes the environment variable hacks for cross-process argument passing
• Reuses server code in unit / integration tests
• Fixes open issues:
  • Delay serving of JS files if there is a pending watch build. #24244 (comment) (cannot lazy-build amp.js in non-default modes
  • 🏗 Lazily build extensions during the default gulp task #24152 (comment) (need to set args in argv and in process.env)
  • 🏗 Add support for pre-building some extensions during gulp --lazy_build #24287 (comment) (unnecessary env vars in server.js)

Coming up:

• Reorganize all dev server files into a separate directory
• Do more with lazy-building of JS files

Partial fix for #24141
Related to #16643, #18635, #24138, #24152, #24199, #24214, #24258, #24244, #24287

[rsimha]

Travis checks are green, and I've tested this in a few different ways. Adding some folks for extra review to spot corner cases that might need follow up fixes.

• @alanorozco The AMP_TESTING env var hack has been removed. Dev dashboard should remain unchanged.
• @estherkim The new server code has been reused for unit and integration tests.
• @calebcordry nodemon and its --inspect flag have been removed. You should now be able to debug this like any othernode process.
• @sparhami Disabling of extension caching has been moved to app.js and exposed via --no_caching_extensions.
• @erwinmombay @zhouyx You folks wrote parts of the original server implementation. Lemme know if you think I've missed something.

[zhouyx]

Thank you @rsimha for improve the server : ) LGTM 👍

Also talked offline, we will need to further improve the app.js code as now the file is getting too big. And the order of the proxy could lead to issues.
For example here: the no-caching-extensions option. Because it's placed at line 843, when we fetched something like /dist/rtv/*/v0/*.js, it won't be applied because of the order.
Or, setHeader() that's placed after the proxy could change the header value.

Also agreed on all request related middleware should go to app.js, while server setting related middleware should go to serve.js.

[rsimha]

Thanks for the detailed review and discussion, @zhouyx! I agree that we should reorganize app.js to make it more predictable and maintainable. Since it's out of scope for this PR, let's deal with it in a follow up. I've filed #24333 to track this work.

Merging this PR now.