MFV r353619: 9691 fat zap should prefetch when iterating

MFV r353619: 9691 fat zap should prefetch when iterating

illumos/illumos-gate@52abb70e073c2a88808c0d66fd810ba8c5080572
https://github.com/illumos/illumos-gate/commit/52abb70e073c2a88808c0d66fd810ba8c5080572

https://www.illumos.org/issues/9691
  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.

Author: Matthew Ahrens <mahrens@delphix.com>
Obtained from:	illumos
MFC after:	2 weeks