HTTP_Router() / serve_http_servers():
- HTTP_Router():
  - Improve documentation
  - Build listeners and routes dynamically from all_listeners
  - set() / unset():
    - add http_server parameter
    - add and remove http_server listeners
  - Rename handler attribute 'name' into 'event'

- serve_http_servers():
  - Rename handler attribute 'name' into 'event'
  - Delegate adding and removing http listeners to HTTP_Router()
  - Share a single HTTP_Router() instance between all upstream servers
  - Prepare code for http_handlers instanceof Pipelet
  - Throw exception if http_handlers is not recognized as valid