security: Change private methods of Validator to protect methods.

The four private methods may be used by sub-classes. For example,
onData & onTimeout are used internally for certificate fetching. Some validators may
need to process received certificate in their styles, e.g. extracting certificate from
some wrappers or try to fetch some other certificate when interest for a certificate times
out. It would be desired to allow sub-classes of Validator to define their own behaviors
in these two methods.

Change-Id: Ibb9fd13937b46376f7834a39ea9f9a3f81980b91