Take objects from the different shard by Policer

[carpawell]

Policer takes objects from the shards one by one and replicates them if needed.

I believe that requesting 200 objects from one shard (== one disk usually) could be slower and less predictable than e.g. requesting 4x50 objects from the 4 different shards (that also could be fetched by 4 different goroutines). I also saw strange disk IOs that could be interpreted as that issue proof (but that should be rechecked for sure):