Broker-Client-API should allow to retrieve query status for all sites

[rwm]

Currently, the broker-client-API can only list/get the sites which already finished calculating a result.
Problem: There is no functionality to determine if/when all results are in and how many sites are processing a query.
Suggestion:
Extend the broker-client-API interfaces to allow one/two more methods

• List Client.getQuerySiteIds(String queryId)
• QueryStatus Client.getQuerySiteStatus(String queryId, String siteId)

alternatively, a different method could return a data structure containing siteIds and status at the same time. e.g. QueryInfo getQueryInfo(queryId)