Junos: family-aware firewall filter reference tracking for family inet6

[dhalperi]

In the process I removed null_filler that was hiding sins, and then
I had to add (and mildly fixup) mtu.

[batfish-bot]

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 72.74%. Comparing base (d1116e0) to head (dd4d94d).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
...fish/grammar/flatjuniper/ConfigurationBuilder.java	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9290      +/-   ##
==========================================
- Coverage   72.75%   72.74%   -0.02%     
==========================================
  Files        3321     3321              
  Lines      170242   170248       +6     
  Branches    20080    20081       +1     
==========================================
- Hits       123863   123847      -16     
- Misses      37206    37225      +19     
- Partials     9173     9176       +3     

Files with missing lines	Coverage Δ
...h/representation/juniper/JuniperStructureType.java	100.00% <100.00%> (ø)
...fish/grammar/flatjuniper/ConfigurationBuilder.java	69.46% <42.85%> (-0.06%)	⬇️

... and 6 files with indirect coverage changes

[progwriter]

Reviewed 3 of 5 files at r1, 5 of 5 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dhalperi)