Initial checkpoint of work for OPENDJ-262: Implement pass through authentication (PTA)

Refactor PasswordPolicy implementation so that we now have a common parent type, "AuthenticatioPolicy", which will act as a common entry point for all authentication policies. In particular, password policies can be thought of as a specific type of authentication policy: one where authentication and the user's credentials are managed locally. PTA can be thought of as a non-local authentication policy: the password is managed by a third party.

With this change we still only have one type of authentication policy and no attempt has been made to refactor code which uses it. More specifically, code such as the local backend WFE just type-casts the authentication policy to a password policy.

Another implication of this change is that sub-entry based password policies now automatically inherit run-time changes made to the default password policy, whereas before a server restart was required.