make sure LazyTableReference isn't resolved until referenced table has been initialised

[dantownsend]

Resolves #757

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 97.18310% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.00%. Comparing base (b13c383) to head (0bd1f12).
⚠️ Report is 247 commits behind head on master.

Files with missing lines	Patch %	Lines
piccolo/columns/base.py	75.00%	1 Missing ⚠️
piccolo/columns/reference.py	96.77%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #758      +/-   ##
==========================================
- Coverage   90.91%   87.00%   -3.92%     
==========================================
  Files         109      109              
  Lines        8214     8257      +43     
==========================================
- Hits         7468     7184     -284     
- Misses        746     1073     +327     

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[dantownsend]

I'll keep this open for now, but improvements were made in #1040, so I'm not sure if this PR is required any more.

[sinisaos]

I'm closing this because it seems to be resolved with #1040. Feel free to reopen if necessary.