MPR-777 Add a per-SteamClient unique identifier message to DebugLog messages (2)

A new PR was merged over at [SteamKit](https://github.com/SteamRE/SteamKit). It should be checked if it's relevant for JavaSteam or not.

Merged at 2019-11-04T11:03:11Z
[Head over to the PRs to see more details.](https://github.com/SteamRE/SteamKit/pulls?q=is%3Apr+is%3Aclosed) (no direct links to avoid reference spamming).