Add automatic file cleanup after task success

[Kobzol]

If a task succeeds, often we do not care about its stdout/stderr output. This PR adds a "file closing mode" to stdout/stderr, which makes HQ automatically remove the files from disk if their task succeeds.

Documentation and changelog isn't done yet, I first want to bakeshed the various names and flags :)

[Kobzol]

I do not like inconsistency between "cleanup" and "delete-if-success". How you want to name keyword for removing empty files, as it will be also kind of "cleanup"?

I agree! But what name should be used on the CLI? Isn't delete-if-finished a bit too long? Maybe it's fine…

[spirali]

Brainstorming:

• Shorten "delete" to just "del", i.e del-if-finished del-if-empty
• Consider the field always a delete rule, so we can just write /my/path/stdout:finished /my/path/stdout:empty
• Emphasis that file will be not there by using "no" prefix: /my/path/stdout:no-finished /my/path/stdout:no-empty
• Variant with "rm": /my/path/stdout:rm-finished /my/path/stdout:rm-empty
• Alternatively "rm-if": /my/path/stdout:rm-if-finished /my/path/stdout:rm-if-empty

[spirali]

Looking at my examples, I'm starting to slightly prefer the last one.

[Kobzol]

Renamed to rm-if-finished and added documentation and changelog.