This allows reducing the state of every statsProcessor by removing pointer to the corresponding statsFunc.
For example, this reduces statsCountProcessor size by 2x.
Previoysly finalizeStats() for some functions such as count_uniq() could run for long periods
of time after the query is canceled, since stopCh wan't propagated to finalizeStats().
Use chunked allocator in order to reduce memory allocations. It allocates objects from slices of up to 64Kb size.
This improves performance for `stats` and `top` pipes by up to 2x when they are applied to big number of `by (...)` groups.
Also parallelize execution of `count_uniq`, `count_uniq_hash` and `uniq_values` stats functions,
so they are executed faster on hosts with many CPU cores when applied to fields with big number
of unique values.
