qlog: rename DefaultTracer to DefaultConnectionTracer

[marten-seemann]

There's a logging.Tracer and a logging.ConnectionTracer. It's confusing that qlog.DefaultTracer returns a ConnectionTracer.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.15%. Comparing base (44e0147) to head (746c9f7).

Files	Patch %	Lines
qlog/qlog_dir.go	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4556      +/-   ##
==========================================
+ Coverage   85.12%   85.15%   +0.03%     
==========================================
  Files         154      154              
  Lines       14841    14843       +2     
==========================================
+ Hits        12633    12639       +6     
+ Misses       1700     1697       -3     
+ Partials      508      507       -1     

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