Prepare TokuDB for 8.0

This is the umbrella task for sub-tasks that can be applied to the PerconaFT repo, and 5.6 and/or 5.7 branches in order to prepare for 8.0. Generally these sub tasks are going to be focused on things like code correctness, clearing new compiler warnings/errors, data structure changes, etc...

8.0 specific tasks will fall under another umbrella specifically for the 8.0 branch.

Richard Prohaska has generously provided several branches as starting points, the first is here https://github.com/percona/percona-server/pull/2116