fix #237, add more context info in logs

fix #237

The branch is based on clockfly:refine_log
To view the change only for this PR, check link https://github.com/clockfly/gearpump/commit/e9f64e008d8a030e41bcd86ee1a68ad6a73f74da

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Maybe add a separator is better, like "/app1/exec2/task1_1"