perf: optimize polling thread and UDP port handling

[jiangdongzi]

1. Optimize polling thread wake-up mechanism:

   • Use condition variable only when polling list is empty
   • Use normal sleep for regular polling interval
   • This improves responsiveness when adding new services

2. UDP port improvements:

   • Support setting UDP port to 0 for automatic port assignment
   • System will automatically assign an available port
   • Actual port number is retrieved using getsockname

This change improves the efficiency of service discovery and makes the UDP port configuration more flexible.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

zhiyuan seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}