Big refactoring of LocalBackendAddOperation, removed nearly 300 lines of code.

LocalBackendAddOperation.java:
Added helper class BooleanHolder.
Made all protected members be private.
Extracted method processAdd() out of the labeled code block in processLocalAdd() + replaced duplicated catch(DirectoryException) with just one.
Extracted method checkHasReadOnlyAttributes() out of processLocalAdd().
Extracted a second method addRDNAttributesIfNecessary() out of addRDNAttributesIfNecessary().
Extracted method checkAttributes() out of checkSchema().