Internal: expose the indices names every action relates to if applicable

Added two new interfaces:
1) IndicesRequest that allows to retrieve the indices the request relates to in a generic manner, together with the indices options that tell how they are going to get resolved and expanded
2) CompositeIndicesRequest for compound requests that hold multiple indices request like MultiSearchRequest, MultiGetRequest, MultiTermVectorsRequest, BulkRequest, BenchmarkRequest, PercolateRequest, MultiPercolateRequest and MoreLikeThisRequest

Taken the chance to streamline the indices options and add them to every request where it makes sense (although they can't be changed from the outside), rather than leaving them implicit in the related TransportAction when indices get expanded (tipycally MetaData#concreteIndices or MetaData#concreteSingleIndex). Added IndicesOptions parameter to MetaData#concreteSingleIndex to make sure it is taken from the request, where the information belongs, instead of hardcoded within MetaData. The concreteSingleIndex method remains but it's just a utility method that returns a single index instead of an array and complains otherwise.

Also made sure NPE is never thrown when setting indices(null) to IndicesAliasesRequest, similar to what SearchRequest does.

Closes #6933