OpenZFS 7638 - Refactor spa_load_impl into several functions

Authored by: Pavel Zakharov <[email protected]>
Reviewed by: Paul Dagnelie <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Matthew Ahrens <[email protected]>
Reviewed by: Andrew Stormont <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tim Chase <[email protected]>
Approved by: Dan McDonald <[email protected]>
Ported-by: Giuseppe Di Natale <[email protected]>

OpenZFS-issue: https://www.illumos.org/issues/7638
OpenZFS-commit: openzfs/openzfs@1fd3785ff6
Closes openzfs#7437