SERVER-106834 is aimed at increasing multiversion test coverage by restarting mongod nodes in different versions each time a node gets killed. During a speculative trial to implement such ticket, some issues arised:

1. When kill primary + should_downgrade are set, first a mongod is shutted down and then the fixture tries to invoke a command on it.
2. When fixing (1), the fixture tries to invoke change_version_if_needed on a list
3. Etc...

Purpose of this ticket is to review/rewrite some parts of the replset fixture and stepdown hook in order to actually work: it looks like a lot of code hanged around for ages without being invoked and it's currently not really usable.

This ticket is a precondition to be able to increase the entropy in existing multiversion suites with kill primary.