Task 2.4.1: Implement efficient Broadcast stream for high concurrency use case

[idky137]

Zaino's internal mempool will need to be able to serve a large number of clients concurrently. For this reason a simple broadcast may not be the best approach. We require lock free read access from an arbitrary number of client tasks and clone free update of the channel.

One possible approach is to use evmap (or possible dashmap) as a lock free map and either a broadcast or watch to alert listeners of updates.

This should be build so it can be used for any efficient broadcast streams required in Zaino (eg. the non-finalised state in #122)