## Description 

This PR implements parallel pruning.
This is necessary for the obj_info pruner, since its pruning is fairly
complicated and expensive. Without pruning multiple ranges in parallel
it cannot catch up.

In this change, we largely maintain the same structure we had before in
the pruner implementation. The primary change is that instead of pruning
the chunks one by one, we prune them in parallel. This does change how
we do accounting on watermarks. To handle it properly as well as deal
with failures and retries, we maintain a map that keeps track of all
pending ranges that need to be pruned. This will guide us in updating
watermark and metrics.

## Test plan 

We probably should add some unit tests for pruner.
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## Release notes

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