YP updates should be serialised per YP directory to isolate problem servers

YP updates (touches) for all mounts and YP directories are performed in series. If a YP server is slow to respond, e.g. 10 seconds, this may cause YP listings to not be maintained properly. E.g., 36 mounts with an average response time of 10 seconds per update will take 6 minutes per update cycle, while the YP directory may expect updates every two to five minutes. All of the YP directories will be affected by one slow YP directory in the configuration.

YP servers should be isolated, with YP updates for multiple YP directories performed in parallel (a separate series for each YP server).