Wrap workspace deployments in a single database transaction

Problem/Motivation

This came up as a review point in #2784921-110: Add Workspaces experimental module.16:

+++ b/core/modules/workspace/src/Plugin/RepositoryHandler/LocalWorkspaceRepositoryHandler.php
@@ -0,0 +1,269 @@
+
+    // Only switch to the target workspace and save entities if there are some
+    // to save.
+    if (!empty($entities)) {
+      // Before saving set the active workspace to the target.
+      $this->workspaceManager->setActiveWorkspace($target_workspace);
+      // Save each revision on the target workspace.
+      foreach ($entities as $entity) {
+        $entity->save();
+      }
+    }

This should happen in a transaction so we can roll back if any of the entity saves fail. Additionally there currently appears to be no protection for saving items in a workspace over more recently updated entities on live - that could happen either via a pre-deploy audit or when saving but seems like we need one or the other?

Note that the second part of the comment was already fixed in #2938626: Do not allow concurrent edits in different workspaces.

Proposed resolution

Do it.

Remaining tasks

User interface changes

API changes

Data model changes