Refactor/split Registry type

What should be done:

Extract messaged handling logic from registry

Also investigate agent status updates when Agent is not found.

Look for this ticket number in code.

Benefit:

Speed-up development

Suggested implementation:

Extract Action/State/etc methods to separate services, as it's done for jobs.

Checkout this PR: https://github.com/percona/pmm-managed/pull/788

What to re-test:

Details: