Deadlock in AsyncLookupDoFn

[RustedBones]

Some dead-lock have been observed on the GrpcDoFn with the following trace

Operation ongoing in step Perform Terminator request/parDo:1 for at least 10m00s without outputting or completing in state finish in thread pool-4-thread-19 with id 78
  at java.base@11.0.20/jdk.internal.misc.Unsafe.park(Native Method)
  at java.base@11.0.20/java.util.concurrent.locks.LockSupport.park(LockSupport.java:194)
  at app//com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:561)
  at app//com.google.common.util.concurrent.AbstractFuture$TrustedFuture.get(AbstractFuture.java:111)
  at app//com.spotify.scio.transforms.FutureHandlers$Guava.waitForFutures(FutureHandlers.java:49)
  at app//com.spotify.scio.transforms.BaseAsyncLookupDoFn.finishBundle(BaseAsyncLookupDoFn.java:199)
  at app//com.spotify.scio.grpc.GrpcDoFn$DoFnInvoker.invokeFinishBundle(Unknown Source)

This indicates one for the future never completes, though the stub was configured with a deadline.

[RustedBones]

Potentially fixed by #5209. Will wait for confirmation before closing