The `FetcherService` wraps the `FetcherState` alongside keeping track
of subscribers for a given fetch.
Typically, this is a channel for which the fetch result should be sent
to.
These subscribers are coalesced by maintaing a map from `(rid, node)`
pairs to the subscriber.
This continues the sans-IO approach where the type of subscribers are
not specified, but rather are kept as a type parameter – to be
specified by the caller.
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