add rate limiting to export and dump commands

[larkost]

When export or dump run they can overwhelm some clusters, slowing down normal traffic. An ultimate solution would be to de-prioritize traffic going to the backup tools, but that would be a lot of work.

A much more approachable solution would be to put rate limiting into the export command (which is used by dump). This would be per-concurrent table, and would have to be something simplistic like number-of-records-per-second, but would probably be the biggest bang-for-the-buck.