To assists debugging e.g cases like #178 (comment) we need to make sure same event-id is set for as many places we log things, as possible.

This issue is only about server.

Example of logs without event-id

• gRPCs connection state changed to 'CONNECTING', etc
• request to https://api.github.com/repos/src-d/lookout/pulls cached"
• http request to /repos/src-d/lookout/pulls","rate-limit":3985
• etc.

Related to previous work in #120 as will require refactoring of some parts to propagate the Context.