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).

contrib/linearize/example-linearize.cfg

@@ -15,3 +15,5 @@ output_file=/home/example/Downloads/bootstrap.dat
 hashlist=hashlist.txt
 split_year=1
 
+# Maxmimum size in bytes of out-of-order blocks cache in memory
+out_of_order_cache_sz = 100000000