Nagios plugin to ensure replication generations are correct

[ezyang]

In the nsds50ruv attribute, replication agreements record a "generation ID" which prevents two masters initialized from different sources from attempting to overwrite each other and cause a mess. Unfortunately, this safety mechanism also means that replication doesn't work, and furthermore, while this will result in a lot of spew to the LDAP error log, this won't show up in the usual status field. We should check that all of the replication generations on our servers are consistent.