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.
Probably, I was following Sprout's examples.  I've changed this (and `auth`) to `mementoDiskUsage` and `mementoAuthProxy` respectively.