Pipeline file opening in FileStream

[thinkharderdev]

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Current FileStream is a state machine that goes from Open (read metadata and build reader) -> Scan (scan decode RecordBatch) in sequence until all files are scanned.

For parquet in particular, the Opener may need to do multiple sequential IO requests:

1. Read footer to get metadata start offset
2. Read footer
3. Fetch page/offset indexes

Seems like it might be possible to parallelize the scan with opening the next file.

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FileStream::Scan adds an Option<FileOpenFuture>. If polled in a scan state, try and pop another file and initialize it's FileOpenFuture (eg try and open the file in parallel) so metadata IO can be done in parallel to scanning of the current file.

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Not do anything

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Not sure if this will be workable but want to experiment with it.

[tustvold]

Fetching metadata in parallel seems like it could yield fairly significant savings, for very little additional memory overhead 👍