David Goulet authored Jan 23, 2017 and Nick Mathewson committed Apr 25, 2017

This commit adds the src/trace directory containing the basics for our tracing
subsystem. It is not used in the code base. The "src/trace/debug.h" file
contains an example on how we can map our tor trace events to log_debug().

The tracing subsystem can only be enabled by tracing framework at compile
time. This commit introduces the "--enable-tracing-debug" option that will
make all "tor_trace()" function be maped to "log_debug()".

Closes #13802

Signed-off-by: David Goulet <dgoulet@torproject.org>