ns_server needs better way to calculate TAP stats.

In order to calculate TAP stats ns_server requests "tapagg _" stats from ep-engine and differentiates them to calculate rates. But the problem with this approach is that ep-engine aggregates stats only for the TAP streams that currently exist. Hence ns_server is not able to account for part of the information because it's completely lost whenever some TAP stream is killed. What's more, it prevented us from implementing more detailed rebalance progress because the estimated number of transferred items was significantly different from the actual one. So we'll probably need some way to keep aggregated stats in ep-engine while rebalance is in progress.