Do not return negative throughput when queue table deleted

[mikeminutillo]

While calculating throughput, when a queue table is deleted, we see a negative throughput entry recorded in the usage report for the day.

This happens because we take two snapshots, an hour apart, and then this to figure out how many messages have been processed:

totalThroughputForHour = totalMessagesEndOfHour - totalMessagesStartOfHour

When a table gets deleted, we do not set the totalMessages which results in the default value of zero. This ends up looking like this:

totalThroughputForHour = 0 - totalMessagesStartOfHour

That is always going to be a negative number, and one that is not representative of the throughput in that hour.

[mikeminutillo]

This is in alignment with how the Endpoint Throughput Counter tool does it.