Only use variable bridge if it is optimal

[blegat]

Caught by SolverTests with ECOS and SCS: https://travis-ci.com/blegat/SolverTests

[codecov-io]

Codecov Report

Merging #973 into master will decrease coverage by <.01%.
The diff coverage is 70%.

@@            Coverage Diff            @@
##           master    #973      +/-   ##
=========================================
- Coverage   95.11%   95.1%   -0.01%     
=========================================
  Files          87      87              
  Lines        9376    9379       +3     
=========================================
+ Hits         8918    8920       +2     
- Misses        458     459       +1

Impacted Files	Coverage Δ
src/Bridges/Variable/single_bridge_optimizer.jl	36.36% <0%> (-3.64%)	⬇️
src/Bridges/graph.jl	100% <100%> (+0.91%)	⬆️
src/Bridges/lazy_bridge_optimizer.jl	96.85% <100%> (+0.02%)	⬆️
src/Bridges/debug.jl	96.74% <100%> (ø)	⬆️
src/Bridges/bridge_optimizer.jl	95.43% <60%> (-0.21%)	⬇️

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