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