Handle low memory conditions

[rillian]

The addrAggregator builds a hash map from wallet ids to associated anonymised IP addresses. We can calculate approximately how much storage this requires for normal traffic, but that doesn't account for traffic spikes. A service should handle that as gracefully as possible, even if under a direct denial-of-service attack.

Two suggestions here:

• Replace the map with something like a bloom filter to bound resource usage.
• Have a background task periodically check process memory use, or the number of elements tracked, and implement some policy (flush? delete data?) when a threshold is reached.