Changes in a nutshell:
* All expression logic is now encapsulated by ExpressionResolver interface.
* MetaData#convertFromWildcards() gets replaced by WildcardExpressionResolver.
* All of the indices expansion methods are being moved from MetaData class to the new IndexNameExpressionResolver class.
* All single index expansion optimisations are removed.
The logic for resolving a concrete index name from an expression has been moved from MetaData to IndexExpressionResolver. The logic has been cleaned up and simplified were was possible without breaking bwc.
Also the notion of aliasOrIndex has been changed to index expression.
The IndexNameExpressionResolver translates index name expressions into concrete indices. The list of index name expressions are first delegated to the known ExpressionResolverS. An ExpressionResolver is responsible for translating if possible an expression into another expression (possibly but not required this can be concrete indices or aliases) otherwise the expressions are left untouched. Concretely this means converting wildcard expressions into concrete indices or aliases, but in the future other implementations could convert expressions based on different rules.
To prevent many overloading of methods, DocumentRequest extends now from IndicesRequest. All implementation of DocumentRequest already did implement IndicesRequest indirectly.
The deleted counter is incremented even if the document is missing. Also, this commit ensures that the scroll id is cleared even if no documents are found by the scan request.
The delete by query plugin adds support for deleting all of the documents (from one or more indices) which match the specified query. It is a replacement for the problematic delete-by-query functionality which has been removed from Elasticsearch core in 2.0. Internally, it uses the Scan/Scroll and Bulk APIs to delete documents in an efficient and safe manner. It is slower than the old delete-by-query functionality, but fixes the problems with the previous implementation.
Closes#7052
