Fix notification task not properly ended

[elmattic]

Summary of changes

After canceling a subscription, a WebSocket client would still get stale responses.

Changes introduced in this pull request:

• Fix Forest sending stale notifications after channel cancelation
• Add debug traces

If you enable debug traces you shoud now see something like:

...
2024-03-01T15:47:19.338708Z  INFO filecoin_proofs::api::winning_post: verify_winning_post:finish    
2024-03-01T15:47:19.363249Z DEBUG forest_filecoin::rpc::channel: sending notification {"jsonrpc":"2.0","method":"xrpc.ch.val","params":[0,{"Type":"apply","Val":[{"Miner":"t017387","Ticket":{"VRFProof":"llZkMUVsIkbv8BJdM5jeE4qqA+3oR6K+QMHqnpikTLCNo7UZ2bJpoCdhYJm/TxWzFbEhahaNXukBbVz1O4keo7YRteUMbn6i34wZ3I1OWrSz/G1klGnE9Ium7oMxrz3A"},...}
2024-03-01T15:47:19.364928Z  INFO forest_filecoin::chain_sync::tipset_syncer: Successfully synced tipset range: [1399384, 1399387]
2024-03-01T15:47:28.203990Z DEBUG forest_filecoin::rpc::auth_layer: Decoded JWT Claims: read
2024-03-01T15:47:28.204116Z DEBUG forest_filecoin::rpc::channel: Got cancel request: id=23
2024-03-01T15:47:28.204162Z DEBUG forest_filecoin::rpc::channel: Sending close message: "{\"jsonrpc\":\"2.0\",\"method\":\"xrpc.ch.close\",\"params\":[0]}"
2024-03-01T15:47:28.204381Z DEBUG forest_filecoin::rpc::channel: Send notification task ended
...

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Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.