Add cleanup method; add support for merged metric files collection

[agoncharov-reef]

Usually built-in mark_process_dead() removes stale *.db metrics files. However, there are cases when it doesn't happen (for example, celery worker dies), and stale files are left. We want to collect & collapse them.

• First, when creating a *.db file, we acquire advisory lock (flock).
• Second, when we perform a cleanup, we select files which are not locked, read them and write combined result into merged_metrics.pkl. If the latter already exists, we combine results with it, too.
• Third, when we collect metrics, we not only read *.db files but also read merged_metrics.pkl and merge everything and then output it.

[agoncharov-reef]

@copilot Check this stuff and make it so nice that everybody immediately wants to raise my salary.

[Copilot]

@agoncharov-reef I've opened a new pull request, #2, to work on those changes. Once the pull request is ready, I'll request review from you.

[agoncharov-reef]

Mf, you should've made it pretty, not show all my ugly mistakes!