Add session keep alive

[bicarbon8]

Changes:

• adds ability to call api client worker periodically with instructions to take a screenshot. this keeps the session active so test runs containing thousands of tests or long running tests will not fail due to session timeout due to inactivity
• allows for overriding default 1 minute delay between keep alive commands to send either more frequently or less frequently
• changes are backwards compatible and the keep alive is disabled by default

Note:

• added benefit of periodic screenshots is that you needn't wait for the test to complete to be able to view the video and instead can check the screenshots to see test status in BrowserStack's dashboard

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[bicarbon8]

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[bicarbon8]

closing this PR based on below feedback from Browserstack:

I have reviewed this internally and I would like to share that while you perform JS testing the entire test is sent as a whole to the browser and hence the screenshot command sent every 60 seconds will not stop the timer for which the browser is idle.
​
Hence, in order to overcome this, you would need to impose some dummy commands like printing some dummy values in the console using JavaScript only or you may execute any JS script to keep your connection alive.
​
Kindly give this a try and let me know how this goes.

so it seems this approach will not work and instead I'll need to change to something that outputs to the browser console

[bicarbon8]

just in case anyone comes across this in the future, the solution that worked for my purposes and didn't require any code changes was to add a timeout: 1800 in the browserStack configuration object of my karma.conf.js file which keeps the session active for 30 minutes.

ex: karma.conf.js

module.exports = function(config) {
  config.set({
    browserStack: {
      username: 'your-username',
      accessKey: 'your-access-key',
      timeout: 1800
    }
    ...
  });
}