Limit number of entries to prefill from cache

[erikjohnston]

Some tables, like device_inbox, take a long time to query at startup for
the stream change cache prefills. This is likely because they are slower
growing streams and so are more fragmented on disk. For now, lets pull
fewer entries out to make startup quicker.

In future, we should add a better index to make it even faster.

[NegativeMjark]

LGTM codewise but I don't think it'll actually fix the perf for device_inbox prefill since the query is doing an index scan rather than a backwards index scan. See query plans #1768.

[erikjohnston]

... but I don't think it'll actually fix the perf for device_inbox prefill...

It does, I tested it.

... since the query is doing an index scan rather than a backwards index scan

Going forwards or backwards only matters if you're counting. When using the stream_id > ? predicate going forwards or backwards is equivalent.

[NegativeMjark]

K, makes sense. (would be really awesome if we could tune this on a worker by worker basis, but I digress).