Reviewer Matt - Memento stats definitions

SNMP MIB updates for the Memento statistics.  I've run through `smilint` and `snmpget` to check it's all valid, then read all the text fields to look for copy-paste errors.
From an implementation perspective, I'm nervous about us defining this under our namespace.  How do we actually find the disk usage?  Iterating through files and counting up their bytes is slow and inaccurate (in particular if you have hard links, as they double-count).  The far more efficient and accurate way is to use separate disk partitions... and if you do that then you can just use the standard UCD-SNMP-MIB table.  Have we designed how we're going to implement this statistic?  (Possibly worth asking Alex because I think he did the design work for this a few months ago.)