Removing singleton pattern from RoutingNodes, as static access to the instance is
unnecessary. Adding javadoc for the method that returns the `RoutingManager`, making
this an instance method as well.

As `RoutingNode#add(MutableShardRouting)` changes the RoutingNodes state, this has to be made private.
All changed need to go through the `RoutingManager` now. Allocators and commands are updated
accordingly.

Shortcut in RoutingAllocation to the RoutingManager.