added take_events() function

[itsmesrds]

Description

This PR adds a new function stream.take_events() which will return events()

https://github.com/robinhood/faust/issues/660 (Fixes #660)

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[fr-ser]

Since returning events is a requirement for manual acknowledgment, how about we include the "fix" for noack-streams as well?

I linked the bug in the feature request for this and here would be a code example of take with consideration for noack https://gist.github.com/fr-ser/347c00e0026cd7941e1d73fd36e54887

[qbzenker]

As a user of Faust, I would also like this PR to be approved/merged. The take_events functionality is super useful.