In this commit, we add a new struct to the package, the chainWatcher.
The duty of this struct is to replace the functionality that was
previously implemented by the closeObserver of each channel. Rather
than the source of notification being tied to the lifetime of a
particular object, it’s now delegated to a persistent object that will
be around for the entire lifetime of the channel (until it’s closed).
This will serve to greatly simplify the code, and eliminate a large
class of bugs.
