core: serialize and deserialize unit start ratelimits

core: serialize and deserialize unit start ratelimits

The logic is taken from dump ratelimit: if the config changes, we discard the
counters. This allows the user apply new limits and "start from scratch" in
that case.

This actually makes StartLimitIntervalSec=infinity (or with a large interval)
work as expected, because the counter is maintained even if daemon-reload
operations are interleaved.