Add option to drop task or flow return values from memory

[zanieb]

Adds a cache_result_in_memory option to flows and tasks to drop a result from memory instead of holding it. You can use this while disabling persistence which is the equivalent of saying "I shall not use this result downstream". By default, using it will enable persistence.

Example

See the examples in the documentation

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• This pull request includes tests or only affects documentation.
• This pull request includes a label categorizing the change e.g. fix, feature, enhancement

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[tpdorsey]

Looks good. Minor comments

[zanieb]

I'm thinking that I should toggle persist_result from None to True when this is used so that the default experience of turning it on is better.

[billpalombi]

Both the proposed names and defaults make sense to me.

[bunchesofdonald]

I like it!