Fix multiple memory allocations

[danko-master]

make test

Before:

ok      github.com/ozontech/ch-rollup/internal/utils/time       1.054s
ok      github.com/ozontech/ch-rollup/pkg/rollup        1.077s

After:

ok      github.com/ozontech/ch-rollup/internal/utils/time       1.020s
ok      github.com/ozontech/ch-rollup/pkg/rollup        1.038s

Fix the problem with the current implementation is multiple memory allocations due to the constant expansion of the result slice via append inside the loop.