Parallel node evictions

[phillebaba]

Currently node-ttl will evict nodes one by one. This is mostly to avoid large amount of node evictions occurring which could cause scaling issues as it would take time to add new nodes to the cluster. This is not really an issue in smaller clusters but a clusters with thousands of nodes may have issue with the slow pace of evictions.

For the future it would be good to evaluate a solution which would evict multiple nodes at the same time.