PHP-736 Test MongoClient construction with tag sets
Fixed PHP-736: MongoClient connection fails if RP tags match nothing

Also fixed it in all other cases, the MongoClient now never actually
throws a MongoConnection exception as in all cases we have a chance of
being able to fullfill the connection requirements (auth, RP, latency,
whatever) on subsequent requests/or userland changes
(->setReadPreferences(), ->selectDB("db2"), ...)