fixes #1313 support deferred nng_aio destruction

[gdamore]

This also converts the HTTP client to make use of this.

[codecov]

Codecov Report

Merging #1371 (2033988) into master (b45f876) will decrease coverage by 0.15%.
The diff coverage is 91.89%.

@@            Coverage Diff             @@
##           master    #1371      +/-   ##
==========================================
- Coverage   79.91%   79.76%   -0.16%     
==========================================
  Files          88       88              
  Lines       19346    19389      +43     
==========================================
+ Hits        15460    15465       +5     
- Misses       3886     3924      +38     

Impacted Files	Coverage Δ
src/supplemental/http/http_client.c	72.68% <40.00%> (ø)
src/core/aio.c	96.50% <100.00%> (-0.19%)	⬇️
src/nng.c	70.56% <100.00%> (+0.07%)	⬆️
src/supplemental/http/http_conn.c	80.70% <0.00%> (-2.18%)	⬇️
src/protocol/pubsub0/pub.c	94.20% <0.00%> (-1.45%)	⬇️
src/core/listener.c	76.63% <0.00%> (-1.41%)	⬇️
src/transport/ipc/ipc.c	79.85% <0.00%> (-1.29%)	⬇️
src/core/dialer.c	82.49% <0.00%> (-0.78%)	⬇️
src/protocol/survey0/survey.c	93.79% <0.00%> (-0.73%)	⬇️
src/transport/inproc/inproc.c	82.24% <0.00%> (-0.73%)	⬇️
... and 9 more

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