Suggestion: Allow pool of already instantiated browser workers

[tehandyb]

Would it be possible to have browser workers already available instead of launching the browser when a new browser worker instance is created? This would allow incoming calls that need a browser, to immediately get it, instead of having to wait for the browser to startup. Similar to how the Page based worker already launches the browser in its init() function. I guess the logic might get a bit complicated on keeping the browser instances around, but I think it could be worth it