Fix system config cache expiration timing

[wxiaoguang]

To avoid unnecessary database access, the cacheTime should always be set if the revision has been checked.

Fix #28057

[lng2020]

Since the first read lock is only used for d.revision and d.cacheTime, why not

 func (d *dbConfigCachedGetter) GetRevision(ctx context.Context) int {
-       d.mu.RLock()
-       defer d.mu.RUnlock()
+       d.mu.Lock()
+       defer d.mu.Unlock()
        if time.Since(d.cacheTime) < time.Second {
                return d.revision
        }
-       d.mu.RUnlock()
-       d.mu.Lock()
        if GetRevision(ctx) != d.revision {
                rev, set, err := GetAllSettings(ctx)
                if err != nil {
@@ -131,8 +129,6 @@ func (d *dbConfigCachedGetter) GetRevision(ctx context.Context) int {
                }
        }
        d.cacheTime = time.Now()
-       d.mu.Unlock()
-       d.mu.RLock()
        return d.revision
 }

to simplify code. I guess it's not very harmful to the performance?

[wxiaoguang]

I think the "read lock" is still good to have.

To simplify it, it could be like the last commit.