When iterating over a ZAP object, we're almost always certain to
iterate over the entire object. If there are multiple leaf blocks, we
can realize a performance win by issuing reads for all the leaf blocks
in parallel when the iteration begins.
For example, if we have 10,000 snapshots, "zfs destroy -nv
pool/fs@1%9999" can take 30 minutes when the cache is cold. This
change provides a >3x performance improvement, by issuing the reads
for all ~64 blocks of each ZAP object in parallel.