couch-upr: make open_connection and sasl_auth public

Hi Volker,

The state defined below is now used to create upr connection to memcached but it is defined in couch_upr.erl instead of hrl file. I have to manually copy it to xdcr code. Can you please move it to header and sharable?

Also, it would be better to rename it to a more specfic name rather than state.

Thanks

Junyi

https://github.com/couchbase/couchdb/blob/upr/src/couch_upr/src/couch_upr.erl#L34-L41