Consolidate the BGSAVE snapshots with AOFRW

[oranagra]

Some day we'll want to completely consolidate the BGSAVE rdb snapshots with AOFRW, so that any bgsave (even one that was created for a replica), can serve as an AOFRW too.

This is now possible since the new multi-part AOF, doesn't have any additional overheads on top of a normal BGSAVE.

This means that there's also no reason for a user to configure both AOF and RDB persistence (we'll block it), so the confusion as to which one to load on startup should be gone too.