prepare for switchover by throttling writes - keep replication lag low

Add prepare switchover behaviour that if necessary makes writing slower so that slaves can catch up. That way, once it’s time to do the switchover, there will be no waiting.

Can this reuse anything from sync_transaction to have new transactions wait or delay some to give time for the replica to catch up. but is it too much to have the entire transaction wait versus adding a fixed amount of ms or ascending. What about throttle filter or other to slow writes down.

make solution smarter a little bit

• existing connections wont have/know they need to throttle, need to atomically switch/update those