Lower message limit for consume script to avoid locking Redis

After some careful benchmarking, it's clear that using the consume
script is more performance intensive than expected. Asking for more
than 1-2 thousand messages when aggregated from multiple channels
causes the Redis process to lock for multiple milliseconds. This will
reduce performance dramatically during replay operations, such as
after multiple consumer failures.

Hopefully we'll be able to find a better way to do channel aggregation
that avoids the performance penalty. For now though, we have to lower
the sustained performance estimate.