Set repl_down_since to zero on state change.
Set repl_down_since to zero on state change.

PR #5081 fixes an "interesting" bug about Redis Cluster failover but in
general about the updating of repl_down_since, that is used in order to
count the time a slave was left disconnected from its master.

While the fix provided resolves the specific issue, in general the
validity of repl_down_since is limited to states that are different
than the state CONNECTED, and the disconnected time is set when the
state is DISCONNECTED. However from CONNECTED to other states, the state
machine must always go to DISCONNECTED first. So it makes sense to set
the field to zero (since it is meaningless in that context) when the
state is set to CONNECTED.

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis