stats: Implement Daily active accounts statistics

[ptrus]

This PR adds /{layer}/stats/active_accounts endpoint, which returns a timeline with daily number of active accounts per layer.

The API only supports daily buckets (the buckets in response are always 86400 seconds big), but it offers overlapping buckets (e.g. 5-minute bucket granularity as in the example below). The window size is fixed to 1 day, while the window step is configurable via config - 5 or 10 minute is likely ok to use in production deployment.

Example responses (on a minimally synced mainnet indexer):

curl http://localhost:8008/v1/consensus/stats/active_accounts?limit=5&window_step_seconds=300
{
    "window_size_seconds": 86400,
    "windows": [
        {
            "active_accounts": 1251,
            "window_end": "2022-04-16T02:35:00Z"
        },
        {
            "active_accounts": 1246,
            "window_end": "2022-04-16T02:30:00Z"
        },
        {
            "active_accounts": 1243,
            "window_end": "2022-04-16T02:25:00Z"
        },
        {
            "active_accounts": 1242,
            "window_end": "2022-04-16T02:20:00Z"
        },
        {
            "active_accounts": 1243,
            "window_end": "2022-04-16T02:15:00Z"
        }
    ]
}

curl http://localhost:8008/v1/consensus/stats/active_accounts?window_step_seconds=30
{"msg":"Invalid format for parameter window_step_seconds: invalid value: 30"}

curl http://localhost:8008/v1/consensus/stats/active_accounts
{
    "window_size_seconds": 86400,
    "windows": [
        {
            "active_accounts": 1230,
            "window_end": "2022-04-16T00:00:00Z"
        },
        {
            "active_accounts": 1326,
            "window_end": "2022-04-15T00:00:00Z"
        },
        {
            "active_accounts": 1367,
            "window_end": "2022-04-14T00:00:00Z"
        },
        {
            "active_accounts": 1119,
            "window_end": "2022-04-13T00:00:00Z"
        },
        {
            "active_accounts": 666,
            "window_end": "2022-04-12T00:00:00Z"
        }
    ]
}

curl http://localhost:8008/v1/emerald/stats/active_accounts?limit=5
{
    "window_size_seconds": 86400,
    "windows": [
        {
            "active_accounts": 3569,
            "window_end": "2022-04-15T00:00:00Z"
        },
        {
            "active_accounts": 3852,
            "window_end": "2022-04-14T00:00:00Z"
        },
        {
            "active_accounts": 3776,
            "window_end": "2022-04-13T00:00:00Z"
        },
        {
            "active_accounts": 2418,
            "window_end": "2022-04-12T00:00:00Z"
        }
    ]
}

TODO:

• Run the updated analyzer on mainnet and sanity check the results
• Likely some code cleanup

[mitjat]

Looks great, thank you!
I much appreciate the comments sprinkled everywhere, and the clean refactoring of existing code where needed.

I have some comments/suggestions, but nothing major.

[ptrus]

Thanks for the thoughtful review, I think I addressed all the comments, please take another look @mitjat

[mitjat]

🎉

Only very minor comments left.