This builds on the concurrent queue to create a generic way to allow
goroutines to pub/sub information. An example includes being notified
each time a state machine is able to carry out a new state transition.
In this commit, we make a new sub-module from the `fn` package. This
lets other projects use it w/o pulling in all of lnd, and also other
sub-packages within lnd.
We also add a temp replace directive for new module. We'll remove this
once we tag the final version.
We start with a simple Map function that can be useful for transforming
objects on the fly.
We may want to eventually make this Taro pakage into a module so we can
just have everything in one place:
https://github.com/lightninglabs/taro/tree/main/chanutils.
