OpenZFS 9284 - arc_reclaim_thread has 2 jobs

OpenZFS 9284 - arc_reclaim_thread has 2 jobs

Following the fix for 9018 (Replace kmem_cache_reap_now() with
kmem_cache_reap_soon), the arc_reclaim_thread() no longer blocks
while reaping.  However, the code is still confusing and error-prone,
because this thread has two responsibilities.  We should instead
separate this into two threads each with their own responsibility:

 1. keep `arc_size` under `arc_c`, by calling `arc_adjust()`, which
    improves `arc_is_overflowing()`

 2. keep enough free memory in the system, by calling
    `arc_kmem_reap_now()` plus `arc_shrink()`, which improves
    `arc_available_memory()`.

Furthermore, we can use the zthr infrastructure to separate the
"should we do something" from "do it" parts of the logic, and
normalize the start up / shut down of the threads.

Authored by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Dan McDonald <danmcd@joyent.com>
Reviewed by: Tim Kordas <tim.kordas@joyent.com>
Reviewed by: Tim Chase <tim@chase2k.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by:  Brad Lewis <brad.lewis@delphix.com>
Signed-off-by: Brad Lewis <brad.lewis@delphix.com>
OpenZFS-issue: https://www.illumos.org/issues/9284
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/de753e34f9
Closes #8165