OpenZFS 9082 - Add ZFS performance test targeting ZIL latency

OpenZFS 9082 - Add ZFS performance test targeting ZIL latency

This adds a new test to measure ZIL performance.

- Adds the ability to induce IO delays with zinject
- Adds a new variable (PERF_NTHREADS_PER_FS) to allow fio threads to
  be distributed to individual file systems as opposed to all IO going
  to one, as happens elsewhere.
- Refactoring of do_fio_run

Authored by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: John Wren Kennedy <jwk404@gmail.com>
OpenZFS-issue: https://www.illumos.org/issues/9082
OpenZFS-commit: https://github.com/openzfs/openzfs/pull/634
External-issue: DLPX-48625
Closes #7491