util: improve debuglog performance

Sample results:

 streams/pipe-object-mode.js n=5000000                        ***     14.74 %       ±1.49% ±1.99%  ±2.60%
 streams/pipe.js n=5000000                                    ***     14.54 %       ±2.10% ±2.79%  ±3.64%
 streams/readable-bigread.js n=1000                           ***      7.85 %       ±1.15% ±1.53%  ±1.99%
 streams/readable-boundaryread.js type='buffer' n=2000        ***     28.70 %       ±1.95% ±2.60%  ±3.40%
 streams/readable-boundaryread.js type='string' n=2000        ***      6.98 %       ±1.55% ±2.07%  ±2.70%
 streams/readable-readall.js n=5000                           ***      7.49 %       ±1.61% ±2.14%  ±2.79%
 streams/readable-unevenread.js n=1000                        ***      9.55 %       ±1.41% ±1.87%  ±2.44%

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines