Proposed fix for AsyncUDP and AsyncUDPPacket

[ssilverman]

Referencing #3287 (comment).

I'll get a pull request done at some point soon, but what do people think of my proposed fix in that comment? It covers both the pass-as-reference and pass-as-copy cases for implementing a AuPacketHandlerFunction. If there is agreement, I'll create a pull request for it.

To summarize the fix:

1. Add a call to pbuf_ref in the AsyncUDPPacket copy constructor, and
2. Call pbuf_free regardless (in AsyncUDP::_recv), outside the if and not in an else.

Here are the steps to show why I think my fix works:

1. The reference count starts at 1, so pbuf_free needs to be called at least once.
2. Case 1: Pass-as-reference version.
   a. packet gets created and the ref count is now 2.
   b. The packet is handled, then the packet destructor gets called and the ref count is back to 1.
   c. pbuf_free is called regardless. The ref count is zero.
3. Case 2: Pass-as-copy version.
   a. packet gets created and the ref count is now 2.
   b. The copy is created and the ref count is now 3.
   c. When the handler ends, both the copy and packet destructors get called and the ref count goes back to 1.
   d. pbuf_free is called regardless. The ref count is zero.

[ssilverman]

See #3290.

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[ssilverman]

Another thought: we could also delete the copy constructor and force use of the reference form of the callback.

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[STALE_SET] This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

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