RESTORE: accept RDB dumps with older versions.

Reference issue #3218.

Checking the code I can't find a reason why the original RESTORE
code was so opinionated about restoring only the current version. The
code in to `rdb.c` appears to be capable as always to restore data from
older versions of Redis, and the only places where it is needed the
current version in order to correctly restore data, is while loading the
opcodes, not the values itself as it happens in the case of RESTORE.

For the above reasons, this commit enables RESTORE to accept older
versions of values payloads.

src/cluster.c

@@ -4535,7 +4535,7 @@ int verifyDumpPayload(unsigned char *p, size_t len) {
 
     /* Verify RDB version */
     rdbver = (footer[1] << 8) | footer[0];
-    if (rdbver != RDB_VERSION) return C_ERR;
+    if (rdbver > RDB_VERSION) return C_ERR;
 
     /* Verify CRC64 */
     crc = crc64(0,p,len-8);