Don't let session commands block new incoming queries

Some driver or framework will encapsulate all queries in prepare statement, it doesn't reuse the prepare stmt, just execute once then close it , then prepare it again;
We found that if there are many backends or some of backend is not health, because session command need route to all, and wait all complete, it will block incoming queries;we already have encountered this situation, that a user use go driver, and one of our backends is slow, so whole cluster is slow;

So we need a way don't let session commands block new incoming queries,