Optimise Topic Service event deduplication

Bug BSERV-11999 describes a problem where deduplication of events on the topic service resulted in the a bottleneck in the Hazelcast event service. In Bitbucket Server 6.7.1, deduplication was disabled on mirror nodes as a temporary workaround to this problem. The approach used for deduplication needs to be improved so this doesn't impact other usages of the topic service, and deduplication should be re-enabled on mirror nodes.