SOLR-18081: ShardRequestTracker should use a unique key for requests

[HoustonPutman]

https://issues.apache.org/jira/browse/SOLR-18081

Instead of indexing results by just the node name, the results should be indexed by node and replica, which is a unique key.

Also, when async is used, the failures and successes should be added just once to the result list. So a success in creating an Async request will not add a success response, it will wait for the actual async status to add that. But a failure in creating an Async request will add a failure response, because there will be no async status to add later. Currently a failed async request will add a success for the creation of the async request, and a failure or the status of the async request, which is very confusing.