Serialize ZTHR operations to eliminate races

Adds a new lock for serializing operations on zthrs.
The commit also includes some code cleanup and
refactoring.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #8229