Shouldn't be necessary to loop over ShardRoutings

As several other classes can change the internal state of the RoutingNodes data structure, inefficient looping over nodes and assigned shards was necessary in the AllocationDeciders.

With larger clusters, reallocation gets too slow. In our current case, we have 5 years of data with daily indices, 6 shards per index, replication factor 1. Recalculating cluster state can take minutes, with the master sitting at 100% CPU in RoutingNodes.shardsRoutingFor( MutableShardRouting ).

The taken approach is
a) making RoutingNodes a singleton, since only one active instance should ever exist anyhow,
b) notifying RoutingNodes of changes in MutableShardRouting instances state.

This certainly is not the most elegant approach and adds complexity instead of removing it, but is what can be done without a major refactoring of allocation.

In the supplied test case execution of the final reallocation is sped up from 22 seconds on my test machine to 4.2 seconds.