Service monitors for tables found in UCD-SNMP-MIB

While most of these could be monitored via a configured threshold, there are a few situations where a "set it and forget it" service monitor would be helpful. Sites with existing configurations for net-snmp should find these useful. The main inspiration for these is that at my site, we have existing configurations for net-snmp process, disk, load and logfile watching, that are managed by puppet, with different thresholds/configs set per machine role. Wanted to migrate from a trap based approach that has been working for years to a service based approach that requires minimal configuration on the onms side. Shamelessly based off the DiskUsageMonitor.