Implement partition processor scheduler

[tillrohrmann]

The partition processor scheduler needs to ensure that enough PartitionProcessors are running wrt the configured ReplicationStrategy. Moreover, it needs to ensure that there is always a single leader for every partition.

### Tasks
- [ ] #1692 
- [ ] #1695 
- [ ] #1696 
- [ ] #1697 
- [ ] #1701 
- [ ] #1703