MFV r266766:

Add a new zfs property, "redundant_metadata" which can have values "all" or
"most".  The default will be "all", which is the current behavior.  When set
to all, ZFS stores an extra copy of all metadata.  If a single on-disk block
is corrupt, at worst a single block of user data (which is recordsize bytes
long) can be lost.

Setting to "most" will cause us to only store 1 copy of level-1 indirect
blocks of user data files.  This can improve performance of random writes,
because less metadata has to be written.  In practice,  at worst about
100 blocks (of recordsize bytes each) of user data can be lost if a single
on-disk block is corrupt.

The exact behavior of which metadata blocks are stored redundantly may change
in future releases.

Illumos issue: 3835 zfs need not store 2 copies of all metadata

MFC after:	2 weeks
Various cleanups and fixes:
- Switch from timeout() to callout_*() for per-request timers.
- Use device_find_child() in the identify routine.
- Use device_printf() instead of passing device_get_nameunit() to
  printf().
- Expand the SBP_LOCK coverage simplifying the locking.
- Uninline STAILQ_FOREACH_SAFE().

Tested by:	sbruno