This is work in progress, focusing on two main areas:
* Avoiding race conditions with cluster configuration propagation.
* Ignoring limitations with redis-cli --cluster fix which makes it hard
to distinguish real errors (e.g. failure to fix) from expected
conditions in this test (e.g. nodes not agreeing on configuration).
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.
