Add an upper bound to the queue size

[kevinburkeshyp]

if the queue fills up with events, it could cause an OOM on the box.

would be better to do this with chpst or similar but on heroku you don't have that luxury.

[kevinburkeshyp]

In addition I'm sure there's some maximum size of events Librato will accept in one request

[kevinburkeshyp]

via Librato support:

On the server-side we allow for payloads up to 3MB, so any number of messages you can fit into that should be successful. However, we do recommend keeping it lower than 1MB as the more messages sent in a payload, the longer that request will generally take.

Any idea how large the items are that get pushed into the aggregator?