merge: polish

[tsibley]

See commit messages.

Checklist

• Tests for table-specific id cols and delims
• Parallel import into separate dbs, then attach them?
• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.

Project coverage is 70.54%. Comparing base (f5275d0) to head (2a3aec0).
Report is 130 commits behind head on master.

Files with missing lines	Patch %	Lines
augur/merge.py	85.71%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1581      +/-   ##
==========================================
+ Coverage   70.51%   70.54%   +0.02%     
==========================================
  Files          78       78              
  Lines        8107     8128      +21     
  Branches     1966     1970       +4     
==========================================
+ Hits         5717     5734      +17     
- Misses       2100     2102       +2     
- Partials      290      292       +2     

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[tsibley]

See #1591.