feat(mdns): supported removal of subtype when updating service (IDFGH-14413)

[tanyanquan]

This MR supports the updating of mdns services when subtypes are changing.

For example, if we start with a client which provides services _sub1,_sub2,_sub3,_sub4, and the client updates itself to now provide services _sub1,_sub5:

In the old implementation, the server only sends out a single new mdns packet with:

• _sub1 (TTL = 120)
• _sub5 (TTL = 120)

However, it is also important to send out a goodbye packet for:

• _sub2 (TTL = 0)
• _sub3 (TTL = 0)
• _sub4 (TTL = 0)

This is similar to the behavior when entire services are removed, to ensure that subtypes are promptly updated as well.

Other tested scenarios include:

• _sub1, _sub2 -> _sub3, _sub4
  • _sub1 and _sub2 (TTL = 0) mdns packet is sent out
• _sub1, _sub2 -> no remaining subtypes
  • _sub1 and _sub2 (TTL = 0) mdns packet is sent out
• no initial subtypes -> _sub3, _sub4
  • no extra packet is sent

heapdiag print has also been used to check for memory leaks, after performing multiple iterations of the above scenarios.

Moreover, any scheduled subtype answers will also be removed from their service to ensure that subsequent tx do not include false information about subtypes which have already been removed.

[CLAassistant]

All committers have signed the CLA.

[david-cermak]

LGTM, just a few nitpicks and questions