Region will not be synced if the channel buffer is full

Bug Report

Lines 1293 to 1298 in e8050f7

    
           if saveKV || needSync { 
        
           	select { 
        
           	case c.changedRegions <- region: 
        
           	default: 
        
           	} 
        
           }

What did you do?

Too large heartbeat qps leads to changedRegions full, and the region miss syncing.

What did you expect to see?

Keep syncing

What did you see instead?

Region miss in the PD follower.

What version of PD are you using (`pd-server -V`)?

Although #9159 can prevent sync tasks from being discarded, it may lead to a significant backlog of tasks. To address this issue more effectively, we need to greatly enhance the consumption capacity of the syncer to avoid task accumulation.

For example, we can directly enable multiple streams for synchronization for each follower.

All is in #9159

syncer: use task runner to sync region (#9159)

If pending tasks accumulate for too long, newly arrived tasks will be regarded as failures. Therefore, we need to set retained=true for the sync runner.