Use cache as long as a trx only performes reads

As before, the cache will be used if there is no ongoing transaction
(includes autocommit being on), or if there is an explicitly read-
only transaction.

In addition, the cache will be used and populated during any other
transaction as long as only pure read statements are executed. After
first non-read statement, the use of the cache is disabled.

Documentation/Release-Notes/MaxScale-2.1.1-Release-Notes.md

@@ -8,6 +8,14 @@ release 2.1.0.
 For any problems you encounter, please consider submitting a bug
 report at [Jira](https://jira.mariadb.org).
 
+## Changed Features
+
+### Cache
+
+The cache will now _also_ be used and populated in a transaction that is
+_not_ explicitly read only, but only until the first statement that modifies
+the database is encountered.
+
 ## Dropped Features
 
 ### MaxAdmin