Update PR #6537 patch to for generality.

After the thread in #6537 and thanks to the suggestions received, this
commit updates the original patch in order to:

1. Solve the problem of updating the time in multiple places by updating
it in call().
2. Avoid introducing a new field but use our cached time.

This required some minor refactoring to the function updating the time,
and the introduction of a new cached time in microseconds in order to
use less gettimeofday() calls.

src/db.c

@@ -1198,8 +1198,12 @@ int keyIsExpired(redisDb *db, robj *key) {
      * blocked to when the Lua script started. This way a key can expire
      * only the first time it is accessed and not in the middle of the
      * script execution, making propagation to slaves / AOF consistent.
-     * See issue #1525 on Github for more information. */
-    mstime_t now = server.lua_caller ? server.lua_time_start : server.cmd_start_mstime;
+     * See issue #1525 on Github for more information.
+     *
+     * Outside the Lua script execution, we use the cached time server.mstime
+     * that is updated before commands executions in call(). */
+    mstime_t now = server.lua_caller ? server.lua_time_start :
+                                       server.mstime;
 
     return now > when;
 }