MM-21328: Fix KVCompareAndSet when new==old (#13612)

`KVCompareAndSet(key, sameValue, sameValue)` can fail spuriously on MySQL if the underlying `UPDATE` requires no actual changes. As per the [MySQL documentation](https://dev.mysql.com/doc/refman/8.0/en/mysql-affected-rows.html), we can't rely on rows affected in this case:

> For UPDATE statements, the affected-rows value by default is the number of rows actually changed. If you specify the CLIENT_FOUND_ROWS flag to mysql_real_connect() when connecting to mysqld, the affected-rows value is the number of rows “found”; that is, matched by the WHERE clause.

It's not tenable to change `CLIENT_FOUND_ROWS` for the all connection, so handle this case in the code instead by running a `SELECT` after the fact. Note that `KVCompareAndSet` has no guarantee of atomicity in this case, but neither would `CompareAndSwap` on which this is method was inspired.

Finally, note that no changes are required for Postgres, which has sane semantics as the default.

Fixes: https://mattermost.atlassian.net/browse/MM-21328

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

i18n/en.json

@@ -6450,6 +6450,14 @@
     "id": "store.sql_oauth.update_app.updating.app_error",
     "translation": "We encountered an error updating the app"
   },
+  {
+    "id": "store.sql_plugin_store.compare_and_set.mysql_select.app_error",
+    "translation": "Failed to query for existing row on MySQL after KVCompareAndSet with unchanged value."
+  },
+  {
+    "id": "store.sql_plugin_store.compare_and_set.too_many_rows.app_error",
+    "translation": "Found more than 1 row on MySQL after KVCompareAndSet with unchanged value."
+  },
   {
     "id": "store.sql_plugin_store.delete.app_error",
     "translation": "Could not delete plugin key value"