Use cached_get_entity_by

Summary

Replace Crawler._get_entity_by... by cached versions of pylib.

Please track on https://gitlab.indiscale.com/caosdb/customers/umg/management/-/issues/84

Test Environment

unit and int tests

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src/doc/concepts.rst

@@ -173,3 +173,14 @@ Example:
 
 File Objects
 ============
+
+TODO
+
+Caching
++++++++
+
+The Crawler uses the cached library function ``cached_get_entity_by``. The
+cache is cleared automatically, when the Crawler does updates, but if you would
+run the same Python process indefinetely the Crawler would not see changes due
+to the Cache. Thus, please make sure to clear the cache if you create long
+running Python processes.