d20d756752 rpc: faster getblockstats using BlockUndo data (Felix Weis)
Pull request description:
Using undo data for a block (rev?????.dat) we can retrieve value information about prevouts and calculate the final transaction fee (rate). This approach is about 80x faster, drops the requirement for `-txindex`, and works for all non-pruned blocks.
```
# 2018-11-25T16:36:19Z Bitcoin Core version v0.17.99.0-edc715240-dirty (release build)
seq 550100 550200 0.00s user 0.00s system 62% cpu 0.004 total
xargs -n1 src/bitcoin-cli getblockstats 0.21s user 0.19s system 17% cpu 2.302 total
# 2018-11-25T16:39:17Z Bitcoin Core version v0.17.0 (release build)
seq 550100 550200 0.00s user 0.00s system 87% cpu 0.002 total
xargs -n1 src/bitcoin-cli getblockstats 0.24s user 0.22s system 0% cpu 3:19.42 total
```
ACKs for commit d20d75:
MarcoFalke:
re-utACK d20d756752
Tree-SHA512: 5babc3eb8d2fee2cb23dc12f522656b80737a540cbf2b13390a8f388304c46c064cca76f896b46a6e2abae8cc582d28e1ab20dd4bb17ad6142f20630c2d30c54