Message store cache should truncate event sets

We already truncate inbound and outbound message sets, but not event sets. We should keep twice as many event keys as message keys, because we expect an ack/nack and possibly one or more delivery report events for each message.
It got removed when I was still trying to do this same thing with events. I can put it back, but my longer-term plan is to make recon cheaper (which requires newer Riak client libraries, so it'll take a while) and remove `switch_to_counters` entirely. Also, the only thing that actually calls this outside of tests is a management command in `vumi-go` that swallows the logging anyway.