Get more random ids at once
Get more random ids at once

in order to work around bug #1767.

This is not ideal because one visitor (or more, in race condition)
out of 500 will have a slow page response.

Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2,Tatoeba/tatoeba2