Utxo counts

[D-Stacks]

Add utxo counts to storage, logs, api and more.

This PR:

1. caches both the pruning point as well as virtual utxo entry counts.
2. adds init code, sanity checks (if enabled) and tests for these counts.
3. exposes them to rpc via get-block-dag-info.
4. signals the expected amount of utxos to sync from during ibd.
5. expands all utxo sync logs to include more sync progress info.
6. adds corresponding logs to the wallet's StateObserver.