MFV r264666:

4374 dn_free_ranges should use range_tree_t

illumos/illumos-gate@bf16b11e8deb633dd6c4296d46e92399d1582df4

MFC after:	2 weeks

(cherry picked from commit 3344ccfa375a376875af16049d67758b63e930ef)
MFV r264667:

4752 fan out read zio taskqs

illumos/illumos-gate@1b497ab83e8f1c58bba5da59c649207a442a4720

(cherry picked from commit aff2f85dda18772982f627879ab3134f575056d8)
MFV r264668:

4754 io issued to near-full luns even after setting noalloc threshold
4755 mg_alloc_failures is no longer needed

illumos/illumos@b6240e830b871f59c22a3918aebb3b36c872edba

MFC after:	2 weeks

(cherry picked from commit b28116674071c0c16c72c5fb110b77592f9f70d3)