trace: Add tracepoints and userspace tracer support

Tor-dev email: https://lists.torproject.org/pipermail/tor-dev/2019-December/014111.html

These are what needs to be done but they sorta need to be together to make sense thus this one ticket:

a. Add a series of tracepoints in tor code base. I propose to start with circuit and cell level tracepoints for now.

b. Add USDT (User Statically-Defined Tracing) probes support which is used by SystemTap, DTrace and perf.

c. Add LTTng support which if enable also emits USDT.

d. Integrate all this to our build system.

About(d), the consensus among the network team is that it should NEVER be enabled in production and should be a configure switch.

I believe if we add on top a torrc option, it might not be that useful in the end considering the configure switch but mainly it will degrade performance since the check needs to be at runtime for every tracepoint.