Mutable collections are causing race conditions

[knocte]

See https://github.com/joemphilips/DotNetLightning/runs/695486967

(We should use Map instead of ConcurrentDictionary.)

[joemphilips]

I'm not 100% sure if the mutable collection is what causing this.
It may be something specific in Infrastructure tests. Anyway, #98 might be a solution if it is too hard to debug.

[knocte]

I'd like to know Andrew's opinion on getting rid of Infrastructure project and tests before we nuke it. However, regardless of that, I think switching to immutable collections is still a worthy change.