cbdatasource needs to SelectBucket after auth to handle when username!=bucketname

Hi Sreekanth,
This was brought up by Adam Fraser originally from trying to use cbdatasource, with auth username/password where the username is not the same as the bucket name.

So, after cbdatasource authenticates the connection, but before it performs an UPROpen(), cbdatasource would need a change to perform a SelectBucket request (see: https://github.com/couchbase/gomemcached/blob/master/mc_constants.go#L88), passing in the bucket name as part of the SelectBucket request.

Also, might be good to flip on the XERRORS flag as part of the first HELO command, as mentioned by Trond Norbye [X], so that the server returns better error messages rather than just disconnecting/hanging-up the connection.

Timeframe/priority is not Drop Everything Now priority, but more like Soon'ish priority to be able to unblock mobile/SyncGateway team.