Improved staging shard performance

using map instead of growing array as shard id can be 1000+ in some cases

domain/consensus/model/staging_area.go

@@ -1,6 +1,8 @@
 package model
 
-import "github.com/pkg/errors"
+import (
+	"github.com/pkg/errors"
+)
 
 // StagingShard is an interface that enables every store to have it's own Commit logic
 // See StagingArea for more details
@@ -19,29 +21,27 @@ type StagingShardID uint64
 // When the StagingArea is being Committed, it goes over all it's shards, and commits those one-by-one.
 // Since Commit happens in a DatabaseTransaction, a StagingArea is atomic.
 type StagingArea struct {
-	shards      []StagingShard
+	shards      map[StagingShardID]StagingShard
 	isCommitted bool
 }
 
 // NewStagingArea creates a new, empty staging area.
 func NewStagingArea() *StagingArea {
 	return &StagingArea{
-		shards:      []StagingShard{},
+		shards:      make(map[StagingShardID]StagingShard),
 		isCommitted: false,
 	}
 }
 
 // GetOrCreateShard attempts to retrieve a shard with the given name.
 // If it does not exist - a new shard is created using `createFunc`.
 func (sa *StagingArea) GetOrCreateShard(shardID StagingShardID, createFunc func() StagingShard) StagingShard {
-	for uint64(len(sa.shards)) <= uint64(shardID) {
-		sa.shards = append(sa.shards, nil)
+	shard, ok := sa.shards[shardID]
+	if !ok {
+		shard = createFunc()
+		sa.shards[shardID] = shard
 	}
-	if sa.shards[shardID] == nil {
-		sa.shards[shardID] = createFunc()
-	}
-
-	return sa.shards[shardID]
+	return shard
 }
 
 // Commit goes over all the Shards in the StagingArea and commits them, inside the provided database transaction.
@@ -52,9 +52,6 @@ func (sa *StagingArea) Commit(dbTx DBTransaction) error {
 	}
 
 	for _, shard := range sa.shards {
-		if shard == nil { // since sa.shards is an array and not a map, some shard slots might be empty.
-			continue
-		}
 		err := shard.Commit(dbTx)
 		if err != nil {
 			return err