Fix or not per-topic limit in the topic table

[harnen]

It's not clear whether we should have a fixed or non-fixed limit per-topic in the topic table.

Arguments for fixed:

1. No need to store more registrants than we return in one go. We try to avoid asking the same registrars multiple times anyway.
2. May help with fairness and avoid unpopular topics struggling to get space (space is the same for all the topics)
3. Simpler table management - we have simple FIFO for each topic
4. May help with load balance (even nodes close to the topic hash won't store more than the limit)

Arguments for non-fixed:

1. Better space utilization. If there's a small number of topics in the system, we might still fill the whole table.
2. We can allocated more space for popular topics within one registrar. (will just need to measure popularity) - With the fixed limits, the popular topics will be present on more registrars, but will take the same amount of space within each registrar.