UTXO snapshot creation (dumptxoutset)

Summary:

• 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

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7945