Proof-of-concept of Spring Boot, Servlet API, and Spring MVC support

[ceefour]

I apologize for so many changes.

This adds support for Spring Boot, Servlet API, and Spring MVC.

• added new handler for Spring Boot (entire loader lifecycle)
• changed handling of servlet in the end of filter chain to better mimic Servlet API behavior, instead of after all Filters have finished executing

Proof of concept is https://github.com/ceefour/wicket8serverless , deployed at https://im9ntgimlc.execute-api.us-east-1.amazonaws.com/prod/

Hopefully this can pave way to official support from Spring Cloud Function, see spring-cloud/spring-cloud-function#107

[sapessi]

Hey @ceefour , like the changes. Can I ask you to send the pull request to the servlet-improvements branch. We need to add unit tests. I'll merge it there, test it, and then push to master.

[ceefour]

@sapessi Thanks for the response, I apologize for not having unit tests. I've edited this request to servlet-improvements branch.

[sapessi]

Thanks @ceefour - I'll merge and work through the unit tests.

[sapessi]

Merging into branch to add unit tests and fixes.