* add unused SnapshotMetadata class
* coinstats: add coins_count
Also changes existing CCoinsStats attributes to be C++11 initialized.
* rpc: add dumptxoutset
Allows the creation of a UTXO snapshot to disk.
* devtools: add utxo_snapshot.sh
to allow easy (if not time-consuming) generation and verification of
snapshots.
* test: add dumptxoutset RPC test
This is a backport of Core PR16899