Put hard upper limits on page size

[dimitarvp]

As @elvanja suggested, we should have some sensible hard upper bound on page_size.

I'd propose 100_000 but even that might be too much. Most programming languages don't cope well with such huge batches. Elixir seems to do fine with 500 to 10_000.

[elvanja]

Whatever number we come up with, it should be configurable / overridable. Probably not that many cases where page size would be that high, but still.