Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r258717: MFV r258371,r258372: 4101 metaslab_debug should allow for
fine-grained control

MFV r258371,r258372: 4101 metaslab_debug should allow for fine-grained control

4101 metaslab_debug should allow for fine-grained control
4102 space_maps should store more information about themselves
4103 space map object blocksize should be increased
4104 ::spa_space no longer works
4105 removing a mirrored log device results in a leaked object
4106 asynchronously load metaslab

illumos/illumos-gate@0713e232b7712cd27d99e1e935ebb8d5de61c57d

Note that some tunables have been removed and some new tunables have
been added. Of particular note, FreeBSD-only knob
vfs.zfs.space_map_last_hope is removed as it was a nop for some time now
(after one of the previous merges from upstream).

MFC after: 11 days
Sponsored by: HybridCluster [merge]