Add release notes for engine optimizations

Change-Id: I9a87c1e2a0bc5a1ba54d7f93f6b1d077088e5023

releasenotes/notes/optimize_adhoc_actions_scheduling-e324f66f962ae409.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    For an ad-hoc action, preparing input for its base action was done more
+    than once. It happened during the validation phase and the scheduling
+    phase. However, input preparation may be expensive in case of heavy
+    expressions and data contexts. This has now been fixed by caching a
+    prepared input within an AdHocAction instance.

releasenotes/notes/use_mapped_entity_for_root_execution-1af6af12ee437282.yaml

@@ -0,0 +1,11 @@
+---
+fixes:
+  - |
+    WorkflowExecution database model had only "root_execution_id" to reference
+    a root workflow execution, i.e. the most parent workflow execution in the
+    execution tree. So if we needed to get an entity itself we'd always make
+    a direct query to the database, in fact, w/o using an entity cache in the
+    SQLAlchemy session. It's now been fixed by adding a normal mapped entity
+    for root workflow execution. In other words, WorkflowExecution class now
+    has the property "root_execution". It slightly improves performance in
+    case this property is accessed more than once per the database session.