Verify (then document) libtrace's threadsafety

[perry]

libtrace should be threadsafe as long as:

• A trace object is only accessed from one thread at a time
• Packets are only modified in one thread, or if it's unmodified, then it can be used by multiple threads.

This should be testing to verify the assumption is correct, then when (successfully!) tested integrated into the wiki and doxygen documentation.