RdWr - readahead - more reable code + fixes + improvements

- readahead should return read length 
- calculate chunk length runtime
RdWr - readahead should return 0 if length is 0
RdWr - test what readahead returns + refactoring
RdWr - readahead long test rewritted using fast_ok
RdWr - readahead simplify branching
RdWr - remove some unneeded code