Decouple SwaggerRouter into HttpHandler and Sequence

[bajtos]

Building on top of the decoupling work that has already landed, build the first full Sequence object to see where we are. Important: this is not the final implementation, just something that works end-to-end and can serve as a good base for further iterations. The few TODOs left in the code are intentional, they'll allow us to land this patch sooner.

I ended up with a new class HttpHandler which provides a method to handle incoming HTTP request. This method composes RoutingTable, Sequence and bits of custom code to invoke the correct controller method in the correct way. I expect these bits to be moved around as we iterate on the Sequence design and implementation.

The existing Application class wraps HttpHandler and provides sugar API for registering controllers. The Server class is requiring a single Application instance now and passes all requests to this single app.

Connect to #308
See also strongloop-archive/loopback-next-sandbox#1

cc @bajtos @raymondfeng @ritch @superkhau @deepakrkris

Tasks:

• initial spike
• fix failing tests
• clean up type definitions, they are all over the place now
• define a new API Application.handleHttpRequest(req, res), move most of the request handling code from Server to Application
• add tests for the newly added code
• review SwaggerRouter tests and move them to appropriate places
• remove SwaggerRouter and its tests

[bajtos]

The pull request is ready for review.

[bajtos]

Hmm, the code coverage is pretty bad, I'll need to add few more tests before this can be landed.

Here are the least covered files:

• packages/core/src/parser.ts
• packages/core/src/sequence.ts
• packages/core/src/server.ts

[raymondfeng]

LGTM

[superkhau]

remove async