Major Description
This is something I noticed while working on SOLR-7271. There are two different router constants:
#1. OverseerCollectionProcessor.ROUTER: represents the argument(s) to the OverseerCollectionProcessor
#2 DocCollection.DOC_ROUTER: represents the router information as stored in the clusterstate in ZK
But these are sometimes used in the other contexts, which can cause issues if the constant values are not the same (as in SOLR-7271).