NXP-14908 improve exception message in class WritePolicyEntryAdaptor

Display the missing field name in the exception message
NXP-14898 MemoryDirectorySession allow to create entry even if it's in read-only mode

Deny to create new entry when the memory diretory is in read-only mode to allow have the same behavior than other directory types in unit tests
NXP-14914 Delete SQLDirectoryProxy and LDAPDirectoryProxy class

Override getReference method to call lazy init method to fix bug when the user Directory try to add reference to group directory