Illumos #4101 metaslab_debug should allow for fine-grained control (partial porting)

Provide an ability to keep all metaslabs in core without loading all metaslabs
when pool is first opened and vice versa.

This commit is partial porting of Illumos #4101
0713e232b7712cd27d99e1e935ebb8d5de61c57d

Signed-off-by: Andrey Vesnovaty <andrey.vesnovaty@gmail.com>