Juggler now implements the same "flatten" protocol as CDP.
This patch:
* copies `Connection.js` from original Puppeteer (with a few renames, e.g. `CDPSesssion` -> `JugglerSession`).
* migrates code to support protocol-level sessions
Support browser target.
Drive-by: switch over to a more devtools'ish protocol:
- use `targetId` instead of `pageId` everywhere
- use target events instead of tab events
This patch refactors Puppeteer-Firefox code to declare public
API in `/lib/api.js` and use it to setup async stack hooks
over the public API method calls.
