MFV r242735:

MFV r242735:

Illumos 13879:4eac7a87eff2:
3329 spa_sync() spends 10-20% of its time in spa_free_sync_cb()
3330 space_seg_t should have its own kmem_cache
3331 deferred frees should happen after sync_pass 1
3335 make SYNC_PASS_* constants tunable

New loader-only tunables:
vfs.zfs.sync_pass_deferred_free
vfs.zfs.sync_pass_dont_compress
vfs.zfs.sync_pass_rewrite

References:
https://www.illumos.org/issues/3329
https://www.illumos.org/issues/3330
https://www.illumos.org/issues/3331
https://www.illumos.org/issues/3335

MFC after:	2 weeks