Add LogStorage utility methods that expose the functionality of BufferedBuildListener, DelayBufferedOutputStream, and GCFlushedOutputStream

[dwnusbaum]

LogStorage implementations that are similar to FileLogStorage need to be able to use the same logic as BufferedBuildListener, DelayBufferedOutputStream, and GCFlushedOutputStream for both performance and correctness reasons, so we should expose these classes' functionality.

I think we can just add two methods to wrap a given OutputStream as needed without having to fully expose the classes. I am not sure what name would be best, feel free to recommend whatever you like.

Testing done

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[jglick]

Allows fixes made for #294 to be used by custom storage systems.

[jglick]

I am not sure what name would be best

The method names chosen seem reasonable to me.