Cache timezone and daylight active flag for safer logging.

With such information will be able to use a private localtime()
implementation serverLog(), which does not use any locking and is both
thread and fork() safe.

src/server.h

@@ -1192,6 +1192,8 @@ struct redisServer {
     int list_compress_depth;
     /* time cache */
     time_t unixtime;    /* Unix time sampled every cron cycle. */
+    time_t timezone;    /* Cached timezone. As set by tzset(). */
+    int daylight_active;    /* Currently in daylight saving time. */
     long long mstime;   /* Like 'unixtime' but with milliseconds resolution. */
     /* Pubsub */
     dict *pubsub_channels;  /* Map channels to list of subscribed clients */