switch to Configuration.getLongBytes for byte options

It's fiddly working out how many bytes to use for 128MB readahead, equally hard to work out what a value actually means.

If we switch to Configuration.getLongBytes() for reading in readahead, partition and threshold values, all existing configs will work, but new configs can use K, M, G suffices.

Easy to code; should add a new test/adapt existing ones.