Matt Ahrens' review comments, round 3.

1) Removed the first-fit allocator.
2) Moved the autotrim metaslab scheduling logic into vdev_auto_trim.
2a) As a consequence of #2, metaslab_trimset_t was rendered superfluous. New
   trimsets are simple range_tree_t's.
3) Made ms_trimming_ts remove extents it is working on from ms_tree and then
   add them back in.
3a) As a consequence of illumos#3, undone all the direct changes to the allocators and
   removed metaslab_check_trim_conflict and range_tree_find_gap.