Prevent updates on the backlog table

[r-marques]

There should be no updates on the backlog table.
We need to create a changefeed to listen to updates in the backlog and if an update occurs revert the change. The tricky part is that several nodes will try to do this a the same and the revert will trigger a new update in itself.

[rhsimplex]

Same problem as #195, I don't think it's merely tricky but actually impossible in the current setup.

[rhsimplex]

Also this is partially incompatible with #198 since transactions will need to be updated periodically with new assignees and fresh timestamps.

[ttmc]

@rhsimplex Can you say a bit about why you think this issue can be closed? Is it because maliciously updated transactions can only become invalid, and therefore will get handled anyhow?

[rhsimplex]

Sorry. #198 requires updates to backlog. And since Byzantine operations will be stopped by some other preventative measure in the future, these handle delete/update to [table] don't really make a whole lot of sense anymore. In fact, with #544 we actually have removed the existing functionality.