7638 Refactor spa_load_impl into several functions

illumos/illumos-gate@1fd3785ff6601d3e391378c2dcbf4c5f27e1fe32

spa_load_impl has grown out of proportions. It is currently over 700
lines long and makes it very hard to follow or debug the import process
even for experienced ZFS developers. The objective is to split it up
in a series of well commented functions.

Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andrew Stormont <andyjstormont@gmail.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Pavel Zakharov <pavel.zakharov@delphix.com>