Change memory resource to be uint64

Just an idea to change the host memory to be uint64.

Currently we used float64 for the memory (which I think doesn't make sense), which leads to a lot of unnecessary casting.

@peterbourgon @jmyounker 
Storage in bytes these days is creeping quickly up to the 10^18/10^19 range, so I think a 64 byte value may be too small.  I'd stay with the floating point for now.