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).
Break into two steps:
* Generate hash list
* Build data file(s) from local bitcoind blocks/ directory.
This supports building one large bootstrap.dat, or multiple
smaller blocks/blkNNNNN.dat files.
