bconf: move switch_to_backup_node to bconf_io

And make it static. This function is only used in bconf_io anyway.

Change-Id: I1f0a43de40436ca96a79aee75db9571f1e24ab65
Reviewed-on: http://review.couchbase.org/29741
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>