[#674] storage engine: Use per-shard worker pools for PUT operation

Make `StorageEngine` to use non-blocking worker pools with the same
(configurable) size for PUT operation. This allows you to switch to using
more free shards when overloading others, thereby more evenly distributing
the write load.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/engine/shards.go

@@ -8,6 +8,7 @@ import (
 	"github.com/nspcc-dev/hrw"
 	"github.com/nspcc-dev/neofs-api-go/pkg/object"
 	"github.com/nspcc-dev/neofs-node/pkg/local_object_storage/shard"
+	"github.com/panjf2000/ants/v2"
 )
 
 var errShardNotFound = errors.New("shard not found")
@@ -29,11 +30,20 @@ func (e *StorageEngine) AddShard(opts ...shard.Option) (*shard.ID, error) {
 		return nil, fmt.Errorf("could not generate shard ID: %w", err)
 	}
 
-	e.shards[id.String()] = shard.New(append(opts,
+	pool, err := ants.NewPool(int(e.shardPoolSize), ants.WithNonblocking(true))
+	if err != nil {
+		return nil, err
+	}
+
+	strID := id.String()
+
+	e.shards[strID] = shard.New(append(opts,
 		shard.WithID(id),
 		shard.WithExpiredObjectsCallback(e.processExpiredTombstones),
 	)...)
 
+	e.shardPools[strID] = pool
+
 	return id, nil
 }