1610643c8b chainparams: add mainnet assumeutxo param at height 840_000 (Sjors Provoost)
Pull request description:
This adds snapshot parameters for mainnet block 840,000.
You can generate the snapshot yourself using `./contrib/devtools/utxo_snapshot.sh` or download my torrent:
* torrent: `magnet:?xt=urn:btih:596c26cc709e213fdfec997183ff67067241440c&dn=utxo-840000.dat&tr=udp%3A%2F%2Ftracker.bitcoin.sprovoost.nl%3A6969`
It would be a good idea to test:
1. That you can produce the same snapshot file, sha256 sum:
```
dc4bb43d58d6a25e91eae93eb052d72e3318bd98ec62a5d0c11817cefbba177b utxo-840000.dat
```
2. That the snapshot works
ACKs for top commit:
fjahr:
re-ACK 1610643c8b
achow101:
ACK 1610643c8b
theStack:
Tested ACK 1610643c8b
mzumsande:
tested ACK 1610643c8b
willcl-ark:
tACK 1610643c8b
Tree-SHA512: 581d8e86379bb044324f04f8559dd0a8946b6e2b145d5f25b38727b30b8cf13d6ac3c8777ff06554d3cf1a072809f7b5fbd693239868578f25dceafe5ba5f57c
0a379a129b