encoding.GetUint64s() returns uninitialized slice, which may contain arbitrary values.
So values in this slice must be reset to zero before using it for counting hits in `uniq` and `top` pipes.
This simplifies pipeProcessor initialization logic a bit.
This also doesn't mangle the original maxStateSize value, which is used in error messages when the state size exceeds maxStateSize.
Change the return values for these functions - now they return the unmarshaled result plus
the size of the unmarshaled result in bytes, so the caller could re-slice the src for further unmarshaling.
This improves performance of these functions in hot loops of VictoriaLogs a bit.