Express/Koa compatibility

[raymondfeng]

Description/Steps to reproduce

LoopBack.next is built with Node core http/https modules. To leverage the ecosystem around Express/Koa, we need to have a compatibility story to cover the following perspectives:

1. API level (req/res/app/...)
2. Middleware

Expected result

Additional information

See #11

[bajtos]

@kjdelisle - PTAL - this is relevant to our discussion about Koa/Express/Micro/Fastify support we had yesterday. I think @virkt25 may be interested too.

[dhmlau]

Looks like this is covered by #1255 (spike) and the other tasks (#1290, #1291, #1292, #1293). Good to close?

cc @raymondfeng @hacksparrow