Onionperf switch for stopping measurements after a specific number of build time events

Hello,

in tpo/core/tor#40157 (closed) we run experiments until 1k circuit build time events are gathered. I tried to automate the experiments by running onionperf with --tgen-num-transfers so that it stops after 1k transfers, but that does not seem to reflect the number of CBT events emited on the control port.

I remember that with --tgen-num-transfers 1000 I only got about 120 BUILDTIMEOUT_SET events on the control port. I will redo this experiment to make sure that I remember right and also check the number of build times on the state file.

\cc @acute @mikeperry @karsten