Add dest mw take two

[aultimus]

This PR adds destination middleware support. This feature was originally released to master but was then reverted as a regression was discovered. That regression was addressed by this commit ( 010219b ) and we are re-releasing.

Verification

Testing completed successfully on local and staging. Run this code in the console of a project connected to a mixpanel destination and then observe the identify event in mixpanel with user id foobarcat

f=function({payload, next, integrations}) {payload.obj.userId = "foo"; next(payload);}; analytics.addSourceMiddleware(f);
f=function(payload, integration, next) {payload.obj.userId = payload.obj.userId + "bar";next(payload);}; analytics.addIntegrationMiddleware(f);
f=function({payload, integration, next}) { payload.obj.userId = payload.obj.userId + "cat";next(payload);}; analytics.addDestinationMiddleware("Mixpanel",[f]);
analytics.identify();

See:
https://www.dropbox.com/s/ncrbwm3tkvphhzf/Screen%20Recording%202020-05-21%20at%202.26.55%20PM.mov?dl=0

Checklist

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• Thorough explanation of the issue/solution, and a link to the related issue
• CI tests are passing
• Unit tests were written for any new code
• Code coverage is at least maintained, or increased.

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[juliofarah]

is there a way, using JSDocs, document the format we expect result to respect?

[aultimus]

I think you would need to do one line per property like this https://jsdoc.app/tags-param.html#parameters-with-properties

[juliofarah]

The code and PR documentation look good to me - will defer the final +1 to someone with more context!
As I don't have a lot of context on AJS as a whole, a newbie question I have is: although this PR is pretty well covered in terms of unit tests, how do we treat end-to-end and integration testing when implementing features like this?

[pooyaj]

Let's fix two comments, otherwise looks good to me.

[pooyaj]

Let's remove the "Revert" line, since it will be i the release log, and confuses people

[aultimus]

resolved by fccfece

[pooyaj]

can we revert this change? I wonder if we need this change even after fixing the emitter issue. Want to make sure all of the old tests are green without any modification.

[aultimus]

resolved by fccfece