Assorted parameter changes for performance tests
- Add async runs for sequential_writes, random_readwrite_fixed and random_writes
- Remove some larger block sizes that give similar results to others
- Remove nthreads == 4 from random_writes_zil test
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>
Closes #12576