Move mstime_t define outside sentinel.c.

The define is now used in other parts of Redis 2.8 tree instead of long
long.

A nice side effect is that now 2.8 and unstable sentinel.c files are
identical as it should be.

src/redis.h

@@ -368,6 +368,8 @@
  * Data types
  *----------------------------------------------------------------------------*/
 
+typedef long long mstime_t; /* millisecond time type. */
+
 /* A redis object, that is a type able to hold a string / list / set */
 
 /* The actual Redis Object */