Internal: moved every single index operation to not replace the index within the original request

Moved get, multi_get, explain, analyze, term_vector, multi_term_vector to not replace the index name with the concrete one within the request.

Most of those requests hold now a concreteIndex, with package private getter and setter, that will be set by the related transport action and serialized over the network. This way the change cn be applied in a backwards compatible manner.

Note: TransportShardSingleOperationAction#resolveRequest doesn't need to replace the index with the concrete one in any case, removed that line of code and left default empty impl. The only actions that don't override the method are in fact shard level actions which already contain the concrete index and don't even need to call concreteSingleIndex. The actions that need to do it already override the method by adding additional logic (e.g. filtered aliases, routing etc.)

Improved tests by randomly introducing the use of an alias, and verifying that the responses always contain the concrete index name.

Also cleaned up TermVectorRequest javadocs and ExplainActionTests and AnalyzeActionTests