Process events with processEventsWhileBlocked() when blocked.

When we are blocked and a few events a processed from time to time, it
is smarter to call the event handler a few times in order to handle the
accept, read, write, close cycle of a client in a single pass, otherwise
there is too much latency added for clients to receive a reply while the
server is busy in some way (for example during the DB loading).

src/redis.h

@@ -1006,6 +1006,7 @@ void disconnectSlaves(void);
 int listenToPort(int port, int *fds, int *count);
 void pauseClients(mstime_t duration);
 int clientsArePaused(void);
+int processEventsWhileBlocked(void);
 
 #ifdef __GNUC__
 void addReplyErrorFormat(redisClient *c, const char *fmt, ...)