research madvise() syscalls to avoid mmap() stalls

[steveyen]

mmap() is terrific, leading to a simple moss implementation.

But, due to the magic of page-faulting in data from disk from mmap(), which is an operation that's invisible to the app, the app thread may suddenly become blocked... and the golang scheduler has no idea it's happening and cannot arrange for the app thread to be used for other needs.

Perhaps various OS API's like madvise() and siblings can help manage this situation in more controlled fashion and reduce page faults.