CLI command to output the total number of (un)opened account versions

[wezrule]

A new CLI command --debug_account_versions has been added which outputs the following on a recent live ledger: (this would automatically include new account versions when they are added)

Opened accounts:
Account version 0 num accounts: 0
Account version 1 num accounts: 1121920

Unopened accounts:
Account version 0 num accounts: 0
Account version 1 num accounts: 59159

Does 60K unopened accounts seem correct? It felt a little low.

However the results weren't like this initially. There were some in version 0, which I found was actually caused by a bug in the upgrade path merge iterator not correctly setting from_first_database in 1 instance. I've fixed it and added various asserts in the v14 upgrade to make sure it is now correct.

I haven't optimised running the new CLI command in any way, takes about a minute in a release configuration.

[guilhermelawless]

Accounts match on on beta-net where all accounts are version 0

# rpc_beta '{"action":"frontiers", "account": "nano_1111111111111111111111111111111111111111111111111111hifc8npp", "count":"1000000"}' | wc -l
727807

# rpc_beta '{"action":"unopened"}' | wc -l
347702

# ./nano_node --network beta --debug_account_versions
Opened accounts:
Account version 0 num accounts: 727803
Account version 1 num accounts: 0

Unopened accounts:
Account version 0 num accounts: 347698
Account version 1 num accounts: 0

4 are extra on the RPCs due to the response format