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().
Previously columns with negative int64 values were stored either as float64 or string
depending on whether the negative int64 values are bigger or smaller than -2^53.
If the integer values are smaller than -2^53, then they are stored as string, since float64 cannot
hold such values without precision loss. Now such values are stored as int64.
This should improve compression ratio and query performance over columns with negative int64 values.
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.
