Speed up shard evacuation process

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when shard evacuation takes more time than it could. We're pulling one object after another and writing them one after another, it's not very productive.

Describe the solution you'd like

Pull objects and push them into write queues (when pushing into multiple shards), accumulate small objects into batches and write batches of objects. Source shard iteration can be done by size if metabase is available.