Allow restarting tenants in parallel

[shmel1k]

Hello! By now, if I set multiple tenants for restart, they will be restarted sequentially, what takes a lot of time

It could be better if some option like --parallel was added. It will allow to restart each tenant independently from other tenant, for example, in different threads

[shmel1k]

We also have to think about limitations of max inflight restarts in order not to ddos state storage