Fix master replica inconsistency for upgrading scenario.

Before this commit, when upgrading a replica, expired keys will not
be loaded, thus causing replica having less keys in db. To this point,
master and replica's keys is logically consistent. However, before
the keys in master and replica are physically consistent, that is,
they have the same dbsize, if master got a problem and the replica
got promoted and becomes new master of that partition, and master
updates a key which does not exist on master, but physically exists
on the old master(new replica), the old master would refuse to update
the key, thus causing master and replica data inconsistent.

How could this happen?
That's all because of the wrong judgement of roles while starting up
the server. We can not use server.masterhost to judge if the server
is master or replica, since it fails in cluster mode.

When we start the server, we load rdb and do want to load expired keys,
and do not want to have the ability to active expire keys, if it is
a replica.

src/rdb.c

@@ -2231,7 +2231,7 @@ int rdbLoadRio(rio *rdb, int rdbflags, rdbSaveInfo *rsi) {
          * received from the master. In the latter case, the master is
          * responsible for key expiry. If we would expire keys here, the
          * snapshot taken by the master may not be reflected on the slave. */
-        if (server.masterhost == NULL && !(rdbflags&RDBFLAGS_AOF_PREAMBLE) && expiretime != -1 && expiretime < now) {
+        if (iAmMaster() && !(rdbflags&RDBFLAGS_AOF_PREAMBLE) && expiretime != -1 && expiretime < now) {
             decrRefCount(key);
             decrRefCount(val);
         } else {

src/server.c

@@ -1691,7 +1691,7 @@ void databasesCron(void) {
     /* Expire keys by random sampling. Not required for slaves
      * as master will synthesize DELs for us. */
     if (server.active_expire_enabled) {
-        if (server.masterhost == NULL) {
+        if (iAmMaster()) {
             activeExpireCycle(ACTIVE_EXPIRE_CYCLE_SLOW);
         } else {
             expireSlaveKeys();
@@ -4863,6 +4863,11 @@ int redisIsSupervised(int mode) {
     return 0;
 }
 
+int iAmMaster(void) {
+    return ((!server.cluster_enabled && server.masterhost == NULL) ||
+            (server.cluster_enabled && nodeIsMaster(server.cluster->myself)));
+}
+
 
 int main(int argc, char **argv) {
     struct timeval tv;

src/server.h

@@ -2393,4 +2393,6 @@ int tlsConfigure(redisTLSContextConfig *ctx_config);
 #define redisDebugMark() \
     printf("-- MARK %s:%d --\n", __FILE__, __LINE__)
 
+int iAmMaster(void);
+
 #endif