Added IHistoricalDataBroker interface.
Split HistoricalDataBroker into two classes: HistoricalDataServer that
handles networking, and HistoricalDataBroker that does the rest (filling
requests from the right sources, etc.)
Added RequesterIdentity property to HistoricalDataRequest, to keep track
of the client identity of the requester.
Added HistoricalDataServerTest class.
Updates to fit the new server/broker class scheme.