Do not use SLF4J/Logback API directly

Motivation:

TrafficShapingHandlerTest uses Logback API directly, which is
discouraged.  Also, it overrides the global default log level, which
silences the DEBUG messages from other tests.

Modifications:

Remove the direct use of Logback API

Result:

The tests executed after TrafficShapingHandlerTest logs their DEBUG
messages correctly.