Add zfs_refcount command.

[PaulZ-98]

Pull request checklist

Please check if your PR fulfills the following requirements:

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)
• [x] Build was run locally and any changes were pushed
• [x] Lint has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• [ ] Bugfix
• [x] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

Issue Number: #192

What is the new behavior?

sdb> spa | member spa_refcount | zfs_refcount
0xffffffffc1089360   dsl_sync_task_common 
0xffff91223d23a000    
0xffff911f9659f000    
0xffff91223f870800    
0xffff91223f870800    
0xffff911f9659c000    
0xffff912249c9d800    
0xffff912249c9d800    
0xffff911f9659c000    
0xffff91223caa6800    
0xffff91223caa6800    
0xffff911f9659c000    
0xffff91223caa4800    

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Other information

Requires a crash dump with reference tracking enabled, so some of the testing infrastructure was refactored.

[codecov-commenter]

Codecov Report

Merging #270 (b1de684) into master (d0cb398) will decrease coverage by 0.21%. The diff coverage is 67.85%.

@@            Coverage Diff             @@
##           master     #270      +/-   ##
==========================================
- Coverage   87.69%   87.48%   -0.22%     
==========================================
  Files          63       64       +1     
  Lines        2577     2605      +28     
==========================================
+ Hits         2260     2279      +19     
- Misses        317      326       +9     

Impacted Files	Coverage Δ
sdb/commands/zfs/zfs_refcount.py	67.85% <67.85%> (ø)

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[PaulZ-98]

I made a version with just the zfs_refcount command and standard test.