don't apply `max_returned_rows` to full table exports

[fgregg]

for labordata.bunkum.us, if have found that users commonly want to download an entire table.

right now, this uses some of the same machinery as arbitrary query exports, including a limit of max_csv_mb.

it would be good if the table exports was not affected by max_csv_mb.

[fgregg]

alternatively, as downloading full tables is a pretty common use case, for the fully static database use case, it could be great to bake out the tables before hand. the downside to that is that if you added both a json and csv version, it would effectively double the storage requirements.