
When replica diskless-load type is swapdb in cluster mode, we didn't backup keys to slots map, so we will lose keys to slots map if fail to sync. Now we backup keys to slots map at first, and restore it properly when fail. This commit includes a refactory/cleanup of the backups mechanism (moving it to db.c and re-structuring it a bit). Co-authored-by: Oran Agra <oran@redislabs.com> (cherry picked from commit 10712afaf3e7f2ea859622fa5b27c96ee8f478c5)