preload: use a MemoryMappedFile instead of a pure read so we can yield every 4K.
preload: use a MemoryMappedFile instead of a pure read so we can yield every 4K.

jmolloy/pedigree,jmolloy/pedigree,jmolloy/pedigree,jmolloy/pedigree,jmolloy/pedigree,jmolloy/pedigree,jmolloy/pedigree