MFV r350898: 8423 8199 7432 Implement large_dnode pool feature

MFV r350898: 8423 8199 7432 Implement large_dnode pool feature

8423 8199 7432 Implement large_dnode pool feature

8423 Implement large_dnode pool feature
8199 multi-threaded dmu_object_alloc()
7432 Large dnode pool feature

llumos/illumos-gate@54811da5ac6b517992fdc173df5d605e4e61fdc0
https://github.com/illumos/illumos-gate/commit/54811da5ac6b517992fdc173df5d605e4e61fdc0
https://www.illumos.org/issues/8423
https://www.illumos.org/issues/8199
https://www.illumos.org/issues/7432

  ZoL issues:
  Improved dnode allocation #6564
  Clean up large dnode code #6262
  Fix dnode_hold() freeing dnode behavior #8172
  Fix dnode allocation race #6414, #6439
  Partial: Raw sends must be able to decrease nlevels #6821, #6864
  Remove unnecessary txg syncs from receive_object() Closes #7197

This updates FreeBSD large_dnode code (that was imported from ZoL) to a version
that was committed to illumos.  It has some cleanups, improvements and fixes
comparing to what we have in FreeBSD now.  I think that the most significant
update is 8199 multi-threaded dmu_object_alloc().

Obtained from:	illumos
MFC after:	3 weeks