Race between calls from resolver and entering idle mode

[dfawley]

From #6778 (comment):

grpc-go/clientconn.go

Line 429 in be1d1c1

cc.resolverWrapper.close()

And ccr.UpdateState's callback calls updateResolverState which needs cc.mu:

grpc-go/clientconn.go

Line 829 in be1d1c1

cc.mu.Lock()

So if UpdateState (and all the things that schedule callbacks?) doesn't check whether the ccr is closing before scheduling callbacks, then we can deadlock.