Add iterator statistics to box.stat.vinyl

[locker]

Absence of aggregated iterator statistics means that one has to iterate over individual indexes to figure out latency/rps of a Vinyl database, which is very inconvenient, because there may be dozens of them. We need to add iterator section to box.stat.vinyl() that would show read stats from all spaces.

[locker]

Needed to investigate performance issue at Mail.Ru FRS (runs 1.10.3)