Subgraphs don't receive notifications of chain head updates so stop indexing

[azf20]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
Sometimes the block stream of synced subgraphs stop getting notifications of chain head updates, so stop indexing. This has been operationally resolved by restarting the indexing nodes. This may be related to the recent tokio upgrade.

What is the expected behavior?
The block stream should continue to get notifications of chain head updates.
As an interim fix, subgraphs could continue to poll for new blocks, even in the absence of a notification