XDCR needs to handle special characters in username and password

When the password for a cluster contains special characters (e.g. '/', ':', '@') goxdcr has issues saving remote cluster reference to that cluster since the following call would fail:

url := fmt.Sprintf("http://%s:%s@%s", remoteUsername, remotePassword, remoteConnectStr)
client, err := couchbase.Connect(url)

Need to escape the remotePassword before passing it to go-couchbase.

The same needs to be done for username as well, which cannot contain "/" but could contain other special chars like "#".

Another issue identified is that goxdcr exited with status 0 rather than 1 when migration service failed.