use LocalAddr for UDP associate bind address

[ge9]

In UDP associate, the IP address used for connection from the client should be notified to client (at least, https://github.com/3proxy/3proxy is implemented in this way).
This also fixes IPv6 address ([::]) replied for IPv4 UDP associate request (in bytes, [5, 3, 0, 1, 0, 0, 0, 0, 0, 0]). Sorry this is wrong.

[ge9]

Oh sorry I found there is another pull request of much the same purpose (#63) but I think my implementation is more appropriate because clients on other hosts require external IP address to use UDP associate bind address.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.71%. Comparing base (8de715f) to head (40c50a9).
Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   63.13%   65.71%   +2.57%     
==========================================
  Files          14       14              
  Lines         784      697      -87     
==========================================
- Hits          495      458      -37     
+ Misses        230      179      -51     
- Partials       59       60       +1     

Flag	Coverage Δ
unittests	65.71% <100.00%> (+2.57%)	⬆️

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