Improve invalidation of stored service password if the endpoint URL is changed (V2)

New version of !1583 (merged), using the same failproof logic but this time mirroring the name and behaviour of the change-tracking methods of ActiveModel::Dirty in order to make it clearer and more natural.

Added more tests to clarify the expected behaviour.

This is an alternative to !1594 (closed)

/cc @vsizov @rspeicher