update alpaca data connector to use Alpaca Data API v2

[jaredmcqueen]

The alpaca data connector, located here: https://github.com/alpacahq/marketstore/tree/master/contrib/alpaca

needs to be updated to use v2 of the realtime streaming spec.

See also: https://github.com/alpacahq/marketstore/issues/579 https://github.com/alpacahq/marketstore/issues/428

It is surprising that polygon works out-of-the-box with marketstore, but not alpaca's own data streaming service. I can help refactor / get working.