increase simultaneous updates for quota test

the nsupdate system test was intermittently failing due to the update quota not being exceeded when it should have been. this is most likely a timing issue: the client is sending updates too slowly, or the server is processing them too quickly, for the quota to fill. this commit attempts to make that the failure less likely by increasing the number of update transactions from 10 to 20.

Closes #3846 (closed)