1. Avoid storing the last evaluation results outside of rules, check for
stale time series as soon as possible;
2. remove duplicated template `Clone()`.
This pull request is primarily reducing memory usage when rules produce
large volumes of results, as seen in
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6894.
The CPU time spent on garbage collection remains high and may be
addressed in a separate PR.