Previously WendelinClient's tracing was controlled by global $trace
variable and was not correlated to current Fluentd log level, or current
plugin log level and thus it was harder for users to use.

Improve it and rework the tracing to use fluentd logging infrastructure.

This way tracing is disabled by default, but can be either globally
enabled with

    fluentd -v -v

or only for wendelin_out plugin via adding

    log_level   trace

to <match ...> section which routes messages to wendelin_out.

/cc @klaus