Add thread-safe Log wrapper

[VReaperV]

The current Log is not thread-safe because the cvar system isn't.

[slipher]

I guess only the log suppression cvars would be a real problem. I used Cvar::GetValue there so that I could use the same cvars and same code in all modules without running into cvar multiple registration issues. We could ifdef this in the engine to read the cvars directly instead of looking them up by name. Then we would only need to read integral cvar values from cvar objects. This is still technically thread unsafe but is probably fine in practice, unlike Cvar::GetValue.

[VReaperV]

Indeed the issue was with the log suppression cvars every time. Reading directly is probably fine as long as the log suppression cvars aren't being modified. On x86 that should work fine™ too.