Switch from using Mutex to Channels for filters

[aakoshh]

consensus-shipyard/fendermint#139 introduces filtering which requires multiple background subscriptions made to the Tendermint web socket API. The state is managed by filter-level Mutexes which need to be taken out for reading and writing, blocking a tokio thread while doing so. This introduces logs of contention on subscriptions that contain OR conditions, as they all require a separate subscription, all racing to take a lock out on the same thing.

Change things so that there is a background process associated with the filter which reacts to read and write commands arriving over a channel, so subscriptions don't become blocked by each other or a reader.