fix: create inFlightSeek on stream reinitialize (#35)

When a stream reconnects, a seek request is made on occasion. We need to set an inFlightSeek, otherwise we run into the following permanent error: https://github.com/googleapis/java-pubsublite/blob/master/google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/internal/wire/SubscriberImpl.java#L211

Author: hannahrogers-google

google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/internal/wire/SubscriberImpl.java

@@ -166,7 +166,11 @@ public void triggerReinitialize() {
  checkArgument(connectedSubscriber.isPresent());
  nextOffsetTracker
  .requestForRestart()
- .ifPresent(request -> connectedSubscriber.get().seek(request));
+ .ifPresent(
+ request -> {
+ inFlightSeek = Optional.of(SettableApiFuture.create());
+ connectedSubscriber.get().seek(request);
+ });
  tokenCounter
  .requestForRestart()
  .ifPresent(request -> connectedSubscriber.get().allowFlow(request));