[#1580] Software Module & Distribution Set lock: add lock at db level (#1643)

1. add _locked_ column in the tables
2. add _locked_ field to the JPA entities
3. add lock set/get at JPA level (with lock DS checking for complete)
4. along with that little reordering of code is made to make it more structured

Signed-off-by: Marinov Avgustin <Avgustin.Marinov@bosch.com>

hawkbit-core/src/main/java/org/eclipse/hawkbit/exception/SpServerError.java

@@ -204,7 +204,7 @@ public enum SpServerError {
      *
      */
     SP_DS_INCOMPLETE("hawkbit.server.error.distributionset.incomplete",
-            "Distribution set is assigned to a target that is incomplete (i.e. mandatory modules are missing)"),
+            "Distribution set is assigned/locked to a target that is incomplete (i.e. mandatory modules are missing)"),
 
     /**
     *