Broker: Subscription matching with many topics brings server to a halt #374
Description
When e.g. connecting 80 test clients subscribing to 10 different topics each (i.e. 800 subscribed topics in total), the broker performance goes down a lot, according to perf/hotspot the subscription matching being at fault. At the same time, subscribing 80 clients to the same 10 topics wasn't an issue at all.
Investigate whether this can be easily optimized, e.g. by caching the matching results, avoiding the topic parsing and matching done again for each and every message. Assuming that both subscriptions are rather stable and the topics of the NOTIFY messages are recurring, that should give a huge improvement.