lua: introduce `Ratelimit` class

Introduce a `Ratelimit` class equivalent to `src/lib/core/ratelimit.h` to
`log` module for internal usage: it acts as a wrapper around the log API.

In scope of #6539 we need to make critical log entries: add S_CRIT alias
for this purpose and a `log_crit` method to `Ratelimit`.

Needed for #6539

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