Improve comments (& avoid a duplicate query) in push actions processing. (#13455)

* Adds docstrings and inline comments.
* Formats SQL queries using triple quoted strings.
* Minor formatting changes.
* Avoid fetching `event_push_summary_stream_ordering` multiple times
  in the same transactions.

Author: clokep

changelog.d/13455.misc

@@ -0,0 +1 @@
+Add some comments about how event push actions are stored.