The test was trying to wait for the replica to start loading the rdb
from the master before it kills the master, but it was actually waiting
for ROLE to be in "sync" mode, which corresponds to REPL_STATE_TRANSFER
that starts before the actual loading starts.
now instead it waits for the loading flag to be set.
Besides, the test was dependent on the previous configuration of the
servers, relying on the fact the replica is configured to persist
(either RDB of AOF), now it is set explicitly.
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>
