Configurable CONCURRENT_ACTIONS

In our environment we have made the constant `CONCURRENT_ACTIONS` a configuration variable as this can significantly improve reload times when you have 8 processes of the same app.

The default behaviour is, and should be, to only do one concurrent action. But for power users and environments where it make sense (e.g. can tolerate the overhead of extra processes running while old workers are finishing their jobs in gracefulReload-scenarios) this should be configurable.