Add metadata column to the migrations table

[benjeffery]

Add migration metadata, note that like edges this is backward compatible as the columns are optional in the kastore.

[codecov]

Codecov Report

Merging #505 into master will decrease coverage by 0.12%.
The diff coverage is 74.71%.

@@            Coverage Diff             @@
##           master     #505      +/-   ##
==========================================
- Coverage   87.08%   86.96%   -0.13%     
==========================================
  Files          21       21              
  Lines       15597    15748     +151     
  Branches     3036     3066      +30     
==========================================
+ Hits        13583    13695     +112     
- Misses       1006     1034      +28     
- Partials     1008     1019      +11     

Flag	Coverage Δ
#c_tests	87.94% <67.69%> (-0.23%)	⬇️
#python_c_tests	90.52% <95.16%> (+0.06%)	⬆️
#python_tests	99.18% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
c/tskit/tables.c	77.74% <63.79%> (-0.42%)	⬇️
python/_tskitmodule.c	84.21% <93.75%> (+0.16%)	⬆️
python/tskit/tables.py	99.80% <100.00%> (+<0.01%)	⬆️
python/tskit/trees.py	98.60% <100.00%> (+<0.01%)	⬆️

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[benjeffery]

Looks like I broke the docs, will fix later as AFK.

[benjeffery]

@jeromekelleher ready for a once over.

[jeromekelleher]

Looks great, thanks @benjeffery. Spotted a few minor typos and a query about whether we want to bump the minor version number.

[benjeffery]

@jeromekelleher Ok, all fixed.

[jeromekelleher]

Merged, thanks. OK, that's the boring stuff out of the way!