MFC r258632,258704: MFV r255255: 4045 zfs write throttle & i/o scheduler
performance work

Sponsored by:	HybridCluster [merge]
MFC r258633: MFV r255256: 3954 metaslabs continue to load even after
hitting zfs_mg_alloc_failure limit
MFC r258634: MFV r258376: 3964 L2ARC should always compress metadata buffers
MFC r258739: zfs mappedread_sf: assert that a page is never partially valid
MFC r258720: MFV r258665: 4347 ZPL can use dmu_tx_assign(TXG_WAIT)