No longer track partially commited element references (#206)

Tracking each unresolved reference to be able to resolve it as soon as
possible has a big impact on memory consumption when there are large
amounts of pending references.
One such case is multiple display styles with hundreds of thousands of
excluded elements.

The transformer now only keeps track of element and aspect ids that were
only partially completed. These elements and aspects will be reprocessed
once all the elements have been processed by the transformer. The only
unresolvable references at that point will be the elements that were
intentionally filtered out by the transformer or the invalid ones.

---------

Co-authored-by: Nick Tessier <[email protected]>

change/@itwin-imodel-transformer-a82348d2-2c15-45c2-9a7c-48d607ef8fce.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "no longer track partially commited element references",
+  "packageName": "@itwin/imodel-transformer",
+  "email": "[email protected]",
+  "dependentChangeType": "patch"
+}

packages/transformer/src/IModelExporter.ts

@@ -158,7 +158,9 @@ export abstract class IModelExportHandler {
     return true;
   }
 
-  /** Called when element is skipped instead of exported. */
+  /** Called when element is skipped instead of exported.
+   * @note When an element is skipped, exporter will not export any of its child elements. Because of this, [[onSkipElement]] will not be invoked for any children of a "skipped" element.
+   */
   public onSkipElement(_elementId: Id64String): void {}
 
   /** Called when an element should be exported.