Add request-id to verbose log messages

[ivanpesin]

This is a proposed enhancement for log messages in verbose mode.

It is difficult to track log messages for a specific request if multiple concurrent requests are being handled by webhook. This PR adds request id string in front of each log message, so it is easy to track relation between log messages and requests.

Example:

[webhook] 2017/09/12 23:10:11 serving hooks on http://0.0.0.0:9000/hooks/{id}
[webhook] 2017/09/12 23:10:17 [e70a0a] incoming HTTP request from [::1]:60390
[webhook] 2017/09/12 23:10:17 [e70a0a] sleep got matched
[webhook] 2017/09/12 23:10:17 [e70a0a] sleep hook triggered successfully
[webhook] 2017/09/12 23:10:17 [e70a0a] executing /tmp/sleep-test.bsh (/tmp/sleep-test.bsh) with arguments ["/tmp/sleep-test.bsh"] and environment [] using /tmp as cwd
[webhook] 2017/09/12 23:10:18 [42e6a9] incoming HTTP request from [::1]:60392
[webhook] 2017/09/12 23:10:18 [42e6a9] sleep got matched
[webhook] 2017/09/12 23:10:18 [42e6a9] sleep hook triggered successfully
[webhook] 2017/09/12 23:10:18 [42e6a9] executing /tmp/sleep-test.bsh (/tmp/sleep-test.bsh) with arguments ["/tmp/sleep-test.bsh"] and environment [] using /tmp as cwd
[webhook] 2017/09/12 23:10:22 [e70a0a] command output: Woke up!
[webhook] 2017/09/12 23:10:22 [e70a0a] finished handling sleep
[webhook] 2017/09/12 23:10:22 200 | 5.015660573s | localhost:9000 | GET /hooks/sleep
[webhook] 2017/09/12 23:10:23 [42e6a9] command output: Woke up!
[webhook] 2017/09/12 23:10:23 [42e6a9] finished handling sleep
[webhook] 2017/09/12 23:10:23 200 | 5.01321799s | localhost:9000 | GET /hooks/sleep

Let me know if you find this useful.

[hassanbabaie]

This would be really nice to have!