Investigate locks in the broker

[AlexGustafsson]

The broker service sets properties on incoming connections. A potential race condition could result in undefined behavior if multiple connections try to access this property at the same time. It might therefore be wise to investigate applying locks on certain properties such as the executorConnection.

In reality, this will likely only be able to occur if quick is used inside of scripts which execute many commands at the same time. That is, it's currently unlikely.