Previously the switch from `split` to `whole` mode had been performed too early,
e.g. when the current cache size became bigger than 1/4 of the allowed cache size.
Now it is performed when the current cache size becomes bigger than 1/2 of the allowed cache size.
This change can reduce memory usage for data ingestion path when big number of active time series are ingested.
This should prevent from cache stats counters going down after cache rotation,
which may corrupt `cache hit ratio` graph on the official Grafan dasbhoards
when using the following query:
1 - (sum(rate(vm_cache_misses_total[5m])) by (type) / sum(rate(vm_cache_requests_total[5m])) by (type))
This should reduce the amount of RAM required for processing time series
with non-zero churn rate.
The previous cache behavior can be restored with `-cache.oldBehavior` command-line flag.
