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.
Synopsis of discussion with Alex:

We have a mechanism in place to destroy the Memento call database if it gets too big (see https://github.com/Metaswitch/memento/blob/dev/memento.root/etc/cron.hourly/memento_disk_usage).  This stat is here so the customer can see how close to that (configured) limit they are.  The proposal is that the script I linked above is enhanced to also report the numbers it calculated (and thus the ones it will use to decide whether to delete data).  This means the statistic only updates every hour, but we anticipate customers having room for 7 days worth of call records (+ headroom) so this doesn't seem like too much of an issue.

On the disk partition proposal, I think it causes many issues in all-in-one/co-located deployments - do we need a separate Cassandra instance per component?  do we try to mount the disks exactly where Cassandra would write the data?  can we change the partitioning scheme easily later to add a new service?