Enhance LND mocking interface to support multiple subscribers

[GeorgeTsagk]

Description

Our current mocking of lnd subscriptions involves single golang channels (e.g the epochChannel) which are used by various lnd subscription calls like RegisterBlockEpochNtfn which currently simply reads from that channel (e.g reading from the epochChannel).

This causes issues when more than one subscribers are listening for updates from a mocked interface, as the order of reading from the channel is not guaranteed by golang.

Solution

We should enhance the internals of such mocked interfaces to instead use channel structures rather than a single channel
e.g

map[int32] chan int32

in order to guarantee exact delivery of updates to all subscribers.