changelist should be able to iter on mounts

Modified changelist_gather()ing for the mountpoint property.
Now instead of iterating on all dataset descendants, we read
/proc/self/mounts and iterate on the mounted descendant datasets only.

Switched changelist implementation from a uu_list_* to uu_avl_* in
order to reduce changlist code-path's worst case time complexity.

Reviewed by: Don Brady <don.brady@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Closes #7967