Simplify spa_sync by breaking it up to smaller functions

The point of this refactoring is to break the high-level conceptual
steps of spa_sync() to their own helper functions. In general large
functions can enhance readability if structured well, but in this
case the amount of conceptual steps taken could use the help of
helper functions.

Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #8293