[assumeutxo] add snapshot data for mainnet block 916'000 and testnet 1'680'000

Summary: Add a more recent UTXO snapshot, leave the previous one as well so if people have the file lying around they can still reuse it.

Test Plan:
Start a bitcoind instance with a fresh datadir, wait for headersync to complete, than load the snapshot

src/bitcoin-cli -datadir=/data0/ecashd_test loadtxoutset /data0/ecashd/utxo_916000

Make sure we reach the tip almost instantly after loading, than let the background sync complete.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18698