Add per-connection threadpool websocket callback executor.

[T4rk1n]

• Add websocket_max_workers argument, default to 4, increase if you have many persistent connections.
• threadpool executor is now on a per connection basis, removing concurrent connection limits on threads, fixes WebSocket callback concurrency is bounded by an unconfigurable thread pool #3825.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud