Record circuit build timeouts

Torperf should record circuit build timeouts, in addition to circuit build times (legacy/trac#2551 (moved)). I think we came up with two subtasks:

1. Learn about the current circuit build timeout and when it changes. Mike says we can add an event handler callback for this.
2. Output this data in some format for subsequent analysis. I think we shouldn't try to squeeze circuit build timeouts into one of the current data formats, but should instead write them to a new file. We need to rethink the data formats anyway, but that's not on the list for the current Torperf sprint.

Mike, want to have a look at this task? It seems you're the person who knows best what needs to be modified for the first subtask.