Allow seeking backwards within a prefetch stream

Linux asynchronous readahead confuses our prefetcher by sometimes making
the stream appear to go backwards, even though the customer is actually
just reading sequentially (awslabs#488). The problem is that with parallel FUSE
threads, the two asynchronous read operations can arrive to the
prefetcher out of order.

This change allows us to tolerate a little bit of backwards seeking in a
prefetch stream. We keep around a little bit of previously read data and
can reload it in the event that a seek goes backwards. We do this by
creating a fake new request containing the rewound bytes, so that the
existing read logic will pick them up. I chose an arbitrary max for the
backwards seek buffer, big enough to handle Linux readahead.

This should fix the readahead issue: in my testing, I no longer saw slow
sequential reads, and the logs confirmed this seeking logic was being
triggered in both directions (forwards and backwards), consistent with
the readahead requests sometimes arriving out of order.