Refactor router

[vooon]

Summary

Refactors router internals for lower lock contention and better runtime efficiency, and switches MAVConn endpoints to a shared ASIO io_service.

Changes

• Reduced lock hold time in router message path:
  • no send operations under router map lock
  • removed goto-style retry flow in routing logic
• Reduced lock hold time in endpoint management:
  • add_endpoint()/del_endpoint() avoid open/close work under router lock
• C++20 cleanup in router:
  • replaced std::bind callbacks with lambdas
  • switched std::shared_timed_mutex -> std::shared_mutex
• Shared IO integration:
  • router now owns a shared mavconn::IoContextRunner
  • all MAVConnEndpoint instances use router shared io_service
  • clean shared IO shutdown in Router destructor

Validation

• Built mavros from /ws
• Ran mavros-router-test
• Passed