Fix average request time with multistreams
When using multistreams, requests are sent in parallel. curl keeps track of different time values for each handle independently. This means that we can retrieve the transfer duration for each handle separatly. It is then possible to calculate the average time taken by each transfer independently. Even so this might be an interesting data to compute, this does not reflect the benefits of using parallelism. Therefore the average time taken by a request is calculate as the total time elapsed to perform all the requests divided by the number of transfers.