Refactor http layer constructs and logic out of the @loopback/rest to @loopback/http-server

[b-admike]

Description

With the introduction of #1082, @loopback/http-server unifies common interfaces for creating endpoints using http/https protocols. This means that @loopback/rest can consume fortified Request and Response objects, and http handlers from concrete implementations of @loopback/http-server such as @loopback/http-server-express and worry only about how to handle controller/route registration, sequence actions, etc.

Acceptance Criteria

[dhmlau]

The PR from @raymondfeng covered most of the requirements already.

[b-admike]

@dhmlau Yes I think so; this story is for the remaining items.

[hacksparrow]

At the moment,http-server-koa is non-functional (which is on the #1082 branch). We need it to be working to compare Express vs. Koa. I am working on it to make it functional.

[dhmlau]

closing as duplicate of #1293.