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 (configurable).