Introduce churn

### Summary
- develop a mechanism that creates churn of a variable rate
- basically, it means to be able to shut down a certain number of random nodes and similarly bring new ones

### Settings
- no churn: the system is completely stable
- moderate churn: nodes have an average lifetime of 4 hours (joins and leaves occur every 15 seconds on average)
- high churn: nodes have an average lifetime of 1 hour (joins and leaves every 3 seconds)