Github-Pull: #5494 Rebased-From: 15de949bb9277e442302bdd8dee299a8d6deee60
Batch up to 10000 requests for a ~30x speedup.
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.