New key type for the disk space accounting rewrite.

 - Holds a variable sized array of u64s (may be more than one for
   accounting e.g. compressed and uncompressed size, or buckets and
   sectors for a given data type)

 - Updates are deltas, not new versions of the key: this means updates
   to accounting can happen via the btree write buffer, which we'll be
   teaching to accumulate deltas.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>