Illumos 5138 - add tunable for maximum number of blocks freed in one txg

Reviewed by: Mattew Ahrens mahrens@delphix.com\
Reviewed by: Adam Leventhal adam.leventhal@delphix.com\
Original author: Max Grossman

References:
  https://www.illumos.org/projects/illumos-gate//issues/5138
  https://reviews.csiden.org/r/85/

Ported by: Turbo Fredriksson <turbo@bayour.com>