Add mnesia multi_dir option to place table files into multiple
subdirectories of mnesia dir.
Move async_dirty tm procs under mnesia_kernel_sup for cleaner shutdown.
Don't queue additional dumps if dump already pending (overload
condition).
Fix default handling of mnesia lib dir width.
Bump ets table lookup hash width from 16 to 9973.
Restructure main and name tabs for efficiency and scale.
Use an initial value for ets key hashing to avoid degenerate behavior
with mnesia frags.
Drop async dirty senders to 3 (also prime number).
Use 17 as modulus for selecting async dirty tm.