Add tracepoint for performance measurements

This is part of sponsor 61 from which we need to measure a series of performance metrics in tor as a baseline and then we'll do the same once we implement congestion control.

Initial list, more might come:

• Record EWMA values (halflife)
• cell transit time in queues at relays
• min/avg/max queue length of circuitmux and channel outbuf
• Time from every 100th cell sent and SENDME response (RTT), for each circuit
• OOM circuit killer being triggered

The plan is to add tracepoints that will be used to record data and from the collected trace(s) we'll compute the various metrics from above like SENDME RTT, cell transit time, and so on.

Parent ticket is #40404