This change introduces a new weighting algorithm to improve metaslab selection.
The new weighting algorithm relies on the SPACEMAP_HISTOGRAM feature. As a result,
the metaslab weight now encodes the type of weighting algorithm used
(size-based vs segment-based).

This also introduce a new allocation tracing facility and two new dcmds to help
debug allocation problems. Each zio now contains a zio_alloc_list_t structure
that is populated as the zio goes through the allocations stage. Here's an
example of how to use the tracing facility:

MSID    DVA    ASIZE      WEIGHT             RESULT               VDEV
   -      0      400           0    NOT_ALLOCATABLE           ztest.0a
   -      0      400           0    NOT_ALLOCATABLE           ztest.0a
   -      0      400           0             ENOSPC           ztest.0a
   -      0      200           0    NOT_ALLOCATABLE           ztest.0a
   -      0      200           0    NOT_ALLOCATABLE           ztest.0a
   -      0      200           0             ENOSPC           ztest.0a
   1      0      400      1 x 8M            17b1a00           ztest.0a

MSID    DVA    ASIZE      WEIGHT             RESULT               VDEV
   -      0      200           0    NOT_ALLOCATABLE           mirror-2
   -      0      200           0    NOT_ALLOCATABLE           mirror-0
   1      0      200      1 x 4M            112ae00           mirror-1
   -      1      200           0    NOT_ALLOCATABLE           mirror-2
   -      1      200           0    NOT_ALLOCATABLE           mirror-0
   1      1      200      1 x 4M            112b000           mirror-1
   -      2      200           0    NOT_ALLOCATABLE           mirror-2

If the metaslab is using segment-based weighting then the WEIGHT column will
display the number of segments available in the bucket where the allocation
attempt was made.