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

[dantownsend]

Resolves #757

[codecov-commenter]

Codecov Report

Merging #758 (30dbaa0) into master (b13c383) will increase coverage by 0.11%.
The diff coverage is 97.18%.

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@@            Coverage Diff             @@
##           master     #758      +/-   ##
==========================================
+ Coverage   90.91%   91.03%   +0.11%     
==========================================
  Files         109      109              
  Lines        8214     8257      +43     
==========================================
+ Hits         7468     7517      +49     
+ Misses        746      740       -6     

Files Changed	Coverage Δ
piccolo/columns/base.py	95.65% <75.00%> (-0.27%)	⬇️
piccolo/columns/reference.py	93.75% <96.77%> (+3.27%)	⬆️
piccolo/apps/migrations/auto/serialisation.py	95.67% <100.00%> (+0.48%)	⬆️
piccolo/columns/column_types.py	88.45% <100.00%> (+0.12%)	⬆️
piccolo/conf/apps.py	91.86% <100.00%> (+0.07%)	⬆️
piccolo/table.py	96.88% <100.00%> (+0.01%)	⬆️

... and 1 file with indirect coverage changes