Fix pool change notification handling in gsi client

The code to handle notifications from /poolsStreaming is racy. It does the following:

1. Subscribe to /poolsStreaming.
2. Once a notification is received, update Indexer list synchronously (it no longer listens to further notifications).
3. Sleep for 1 second. Repeat Step1.

During the sync processing in Step2 and sleep in Step3, any number of notifications can be missed.