Fix inefficient rewrite flushing

[westonruter]

Fixes #1075

[westonruter]

Build for testing: pwa.zip

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (e034397) 29.34% compared to head (eadf98d) 29.45%.

Files	Patch %	Lines
wp-includes/class-wp.php	0.00%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #1096      +/-   ##
=============================================
+ Coverage      29.34%   29.45%   +0.10%     
  Complexity       334      334              
=============================================
  Files             57       57              
  Lines           1690     1684       -6     
=============================================
  Hits             496      496              
+ Misses          1194     1188       -6     

Flag	Coverage Δ
php	29.45% <0.00%> (+0.10%)	⬆️
unit	29.45% <0.00%> (+0.10%)	⬆️

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