multi endpoint flight retrieval capability

This adds the capability to fetch data from multiple endpoints, as specified by the server. This is handled by a new multi-threaded fetch_flights function that can connect to multiple endpoints simultaneously, yielding the stream data from each in an interleaved manor.

The various supported arrakis methods are modified to use this new flight retrieval function. All arrakis methods other than count and fetch are also modified to be generators of the data streamed from the endpoints. This should allow for faster retrieval of data in most cases.

don't squash, review commits individually