Improve ETA

[Joacchim]

Currently, the ETA measures the last few seconds of bandwidth in order to attempt an estimation at how much data can be transferred. It does not take into account:

• Bandwidth variation/evolution on a larger timespan than 10 secs.
• Small objects migrations that induce a lot of latency (and thus reduce bandwidth)

We should try to find an efficient ETA algorithm, make it testable, run scenarios with it, and replace the current, unreliable one.