We risked deadlocking on shutdown if a client (in our case a contract
resolver) attempted to schedule a sweep of an input after the
ChainNotifier had been shut down. This would cause the `collector`
goroutine to exit, and not handle incoming requests, causing a deadlock
(since the ChainArbitrator is being stopped before the Sweeper in the
server).
To fix this we could change the order these subsystems are stopped, but
this doesn't ensure there aren't other clients that could end up in the
same deadlock scenario. So instead we keep handling the incoming
requests even after the collector has exited (immediatly returning an
error), until the sweeper is signalled to shutdown.
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