Allow Pub/Sub in contexts where other commands are blocked.

Redis loading data from disk, and a Redis slave disconnected from its
master with serve-stale-data disabled, are two conditions where
commands are normally refused by Redis, returning an error.

However there is no reason to disable Pub/Sub commands as well, given
that this layer does not interact with the dataset. To allow Pub/Sub in
as many contexts as possible is especially interesting now that Redis
Sentinel uses Pub/Sub of a Redis master as a communication channel
between Sentinels.

This commit allows Pub/Sub to be used in the above two contexts where
it was previously denied.

src/redis.h

@@ -85,6 +85,8 @@
 #define REDIS_CMD_NOSCRIPT  64              /* "s" flag */
 #define REDIS_CMD_RANDOM 128                /* "R" flag */
 #define REDIS_CMD_SORT_FOR_SCRIPT 256       /* "S" flag */
+#define REDIS_CMD_LOADING 512               /* "l" flag */
+#define REDIS_CMD_STALE 1024                /* "t" flag */
 
 /* Object types */
 #define REDIS_STRING 0