Compute aggregate transfer success rate stats from the trace

[carver]

Derive some statistics, by analyzing the query traces from all different audit strategies.

Derive the transfer fail rate as a percentage of attempted transfers. Break that down by:

• client type & version
• failure type (utp vs validation, etc)
• content type

We also probably want to keep a list of recent failures, per client. It doesn't need to be exhaustive. Just a few examples per client is sufficient (maybe per client version).