fn: add new ConcurrentQueue[T] data structure

This is a version of the queue we have elsewhere, but we can get rid of
all the casting and interface usage in favor of a type param.

fn/go.mod

@@ -1,3 +1,5 @@
 module github.com/lightningnetwork/lnd/fn
 
 go 1.19
+
+require github.com/lightninglabs/neutrino/cache v1.1.2