Add a fiber responsible for periodic re-training of compression dictionaries
(for tables which opted into dict-aware compression).
As of this patch, it works like this:
every `$tick_period` (15 minutes), if we are the current Raft leader,
we check for dict-aware tables which have no dict, or a dict older
than `$retrain_period`.
For those tables, if they have enough data (>1GiB) for a training,
we train a new dict and check if it's significantly better
than the current one (provides ratio smaller than 95% of current ratio),
and if so, we update the dict.
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