[JSONRPC] Fix websocket subscription

[dbrajovic]

Fixes EDGE-387

Description

This PR fixes the issue of websocket subscriptions becoming unresponsive after a predefined timeout.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

Use the websocat tool to verify the subscription is sending responses:

1. Start a 4-node cluster
2. Connect to a node using websocat [node_ws_url]
3. Send the request (in websocat):
   {"id": 1, "method": "eth_subscribe", "params": ["newHeads"]}
4. Observe the responses are being received (subscription is not becoming silent)

[0xAleksaOpacic]

Great job. Thank you manual testing instructions

[zivkovicmilos]

Looks good, thank you for hunting down this bug 🙏

I've left some comments regarding the test that we should discuss 👍

[Kourin1996]

Looks good to me