Investigate making MaxOnionsPending time-based

A lot of relays seem to be failing circuits due to too many onionskins right now. Are there really too many? A machine with NumCPUs 8 might be able to churn through 100 onionskins pretty quickly. Perhaps we should consider making the queue limited based on time-to-process rather than a fixed count.

Nick says the first step is altering the "Your computer is too slow" log message to output a queue processing time to see how long most nodes are spending on their queues, to decide if this is worth the effort to change.