messaging: add new priority field

[derekperkins]

As discussed on Slack with @sougou, we decided to add a new required priority field. This PR adds the field to the tests and modifies the load query + index to order by priority and then time_next.

epoch remains as is, it just won't be used to prioritize. instead you can introduce a new priority column, and that will be used instead of epoch (both for the select query and in the cache)
https://vitess.slack.com/archives/C9EDPDJT1/p1584943246101100

[derekperkins]

Things I considered while doing this:

• Restricting the priority column to uint8: There's not a great reason to need more than 255 levels of priority. At the end of the day, individuals can do that themselves, and I just left it as an int64 in the Vitess code
• Dealing with NULL values: I thought about using IFNULL(priority, 0) to deal with null values, but I didn't think that vstream would support that.

[sougou]

Nice work!