Skip low level message to avoid clock_gettime

[Laisky]

Skip lower level logs to avoid too muchtime.Now(), especially each time.Now() will invoke clock_gettime. This costs a lot of time even though there is no log match the level to print.

This fix will significant reduce the number of syscall and improve performance.

On my test (set log level as INFO, then invoke logger.Debug("xxx") in benchmark ):

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #671 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #671      +/-   ##
==========================================
+ Coverage   97.38%   97.38%   +<.01%     
==========================================
  Files          40       40              
  Lines        2102     2107       +5     
==========================================
+ Hits         2047     2052       +5     
  Misses         47       47              
  Partials        8        8

Impacted Files	Coverage Δ
logger.go	94.73% <100%> (+0.24%)	⬆️

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[fgrosse]

There was some discussion around the topic in #664 . The issue with the change is that this would break compatibility for sample based logging cores. This is however on the wishlist for 2.0 (#388)