Reenable backtraces where appropriate

The oboe_log function used to generate a backtrace every time it was called. The good news is that it can now suppress backtrace generation with an additional bool parameter! The bad news is that the default setting, off, differs from how the module used to work. I'm filing this as a bug accordingly.

Rather than setting the flag to TRUE everywhere to restore old behavior, we should consider only collecting backtraces in places where it's likely to be interesting. For instance: not on info events, or on hook-based layer exits.