Scripting: use mstime() and mstime_t for lua_time_start.

server.lua_time_start is expressed in milliseconds. Use mstime_t instead
of long long, and populate it with mstime() instead of ustime()/1000.

Functionally identical but more natural.

src/redis.h

@@ -762,8 +762,8 @@ struct redisServer {
     redisClient *lua_client;   /* The "fake client" to query Redis from Lua */
     redisClient *lua_caller;   /* The client running EVAL right now, or NULL */
     dict *lua_scripts;         /* A dictionary of SHA1 -> Lua scripts */
-    long long lua_time_limit;  /* Script timeout in seconds */
-    long long lua_time_start;  /* Start time of script */
+    mstime_t lua_time_limit;  /* Script timeout in milliseconds */
+    mstime_t lua_time_start;  /* Start time of script, milliseconds time */
     int lua_write_dirty;  /* True if a write command was called during the
                              execution of the current script. */
     int lua_random_dirty; /* True if a random command was called during the