Cache dataOffset and symlink when requesting zip files

Currently Readlink and dataOffset are evaluated each time, we should cache them to lower the latency when serving from zip.

To ensure that Readlink is processed in a predictable time, using minimal amount of requests, especially that in some cases we need to perform symlink traversal.

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The following discussion from !348 (merged) should be addressed:

• @ayufan started a discussion:

  I wish that we would cache dataOffset, since the file.DataOffset() each time makes us to fire additional request.