grpc-js-xds: Reference clusters for ConfigSelector lifetime

[murgatroid99]

First, this modifies the ConfigSelector API to make it an object instead of a function. The object has an invoke method, which is called the same way the function was previously called, and an additional unref function, which signals the end of the lifetime of the ConfigSelector. This allows resolvers to track and act on the lifetime of the ConfigSelector.

Second, this modifies the xDS resolver so that it keeps a cluster ref for each cluster that a ConfigSelector can select, for the lifetime of the ConfigSelector, plus an additional ref for each call for which a cluster has been selected, until that call is committed. This ensures that once a ConfigSelector is created, every cluster it can select will be subscribed to continuously until no more calls need it.