contrib: make linearize-data.py cope with out-of-order blocks
Make it possible to read blocks in any order. This will be required
after headers-first (#4468), so should be merged before that.
- Read block header. For expected blocks, continue, else skip.
- For in-order blocks: copy block contents directly. Write prior
out-of-order blocks if this connects a consecutive span.
- For out-of-order blocks, store extents of block data for later
retrieval. Cache out-of-order blocks in memory up to 100MB