Store the explain plan and query text in benchto

[cawallin]

When further investigating slow queries, it is very valuable to have the explain plan to see what changed. It's also not possible from the benchto UI to see what the text of the query is.

EXPLAIN (type=distributed) would be most helpful.

At least initially, it's ok if it's just stored in the benchto database. However, eventually either it needs to make its way to the benchto UI, or the benchto database needs to be cleaned up so that it's easier to find things.