transformer workaround memory leak when processing large models (#3253)

- see this issue report of the leak nodejs/node-addon-api#1140
- the leak appears to be a bug in node[-addon-api]. Tracing the gc shows JS heap size remains stable and old-generation garbage collection sweeps occur in parallel with the JS work, but objects that use `Napi::ObjectWrap` hold on to some memory until the thread yields. See the above issue for more details.
- the workaround is to force yield the javascript job in long loops
- the transformation service already employs this workaround to handle huge models

Co-authored-by: Michael Belousov <MichaelBelousov@users.noreply.github.com>

common/changes/@itwin/core-transformer/transformer-handle-large-models_2022-02-22-18-07.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@itwin/core-transformer",
+      "comment": "mitigate memory leak in large model processing",
+      "type": "none"
+    }
+  ],
+  "packageName": "@itwin/core-transformer"
+}

core/transformer/src/IModelExporter.ts

@@ -475,6 +475,7 @@ export class IModelExporter {
       }
       while (DbResult.BE_SQLITE_ROW === statement.step()) {
         await this.exportElement(statement.getValue(0).getId());
+        await new Promise(setImmediate); // workaround for: https://github.com/nodejs/node-addon-api/issues/1140
       }
     });
   }
@@ -638,6 +639,7 @@ export class IModelExporter {
         const relInstanceId: Id64String = statement.getValue(0).getId();
         const relProps: RelationshipProps = this.sourceDb.relationships.getInstanceProps(baseRelClassFullName, relInstanceId);
         await this.exportRelationship(relProps.classFullName, relInstanceId); // must call exportRelationship using the actual classFullName, not baseRelClassFullName
+        await new Promise(setImmediate); // workaround for: https://github.com/nodejs/node-addon-api/issues/1140
       }
     });
   }

core/transformer/src/IModelTransformer.ts

@@ -709,7 +709,7 @@ export class IModelTransformer extends IModelExportHandler {
     }
     const aspectDeleteIds: Id64String[] = [];
     const sql = `SELECT ECInstanceId,Identifier,JsonProperties FROM ${ExternalSourceAspect.classFullName} aspect WHERE aspect.Scope.Id=:scopeId AND aspect.Kind=:kind`;
-    this.targetDb.withPreparedStatement(sql, (statement: ECSqlStatement): void => {
+    await this.targetDb.withPreparedStatement(sql, async (statement: ECSqlStatement) => {
       statement.bindId("scopeId", this.targetScopeElementId);
       statement.bindString("kind", ExternalSourceAspect.Kind.Relationship);
       while (DbResult.BE_SQLITE_ROW === statement.step()) {
@@ -722,6 +722,7 @@ export class IModelTransformer extends IModelExportHandler {
           }
           aspectDeleteIds.push(statement.getValue(0).getId());
         }
+        await new Promise(setImmediate); // workaround for: https://github.com/nodejs/node-addon-api/issues/1140
       }
     });
     this.targetDb.elements.deleteAspect(aspectDeleteIds);