Deprecate KerberosToken constructor that exposes UserGroupInformation

KerberosToken works by setting up some metadata about a current user to be validated later by when a UGIAssumingTransport is created. The public KerberosToken(String principal, File keytab, boolean replaceCurrentUser) throws IOException constructor leaks out the UserGroupInformation (or a construct with a similar concept of having mutable, static variables) dependency, by allowing users to login as a user and hold on to their credentials in a place external to the token itself.

While working on ACCUMULO-4306, I was thinking about whether or not this feature/side effect of KerberosToken made sense. It is a user convenience thing, but it's not a big convenience, as logging in as another user isn't complicated (via UserGroupInformation or standard Java methodologies. It's also a potential hazard if two threads update the same UserGroupInformation class at the same time.

Without much convenience and concurrency pitfalls, marking this deprecating in 1.8 and removing in 2.0 makes the most sense.