start upnp per listen_socket_t

[arvidn]

This simplifies the UPnP class to only be responsible for a single subnet.

[codecov-io]

Codecov Report

Merging #4262 into RC_1_2 will increase coverage by 0.18%.
The diff coverage is 55.31%.

@@            Coverage Diff            @@
##           RC_1_2   #4262      +/-   ##
=========================================
+ Coverage   74.31%   74.5%   +0.18%     
=========================================
  Files         438     438              
  Lines       60633   60681      +48     
  Branches     8953    8967      +14     
=========================================
+ Hits        45058   45208     +150     
+ Misses      11101   10973     -128     
- Partials     4474    4500      +26

Impacted Files	Coverage Δ
include/libtorrent/upnp.hpp	78.26% <ø> (ø)	⬆️
include/libtorrent/enum_net.hpp	6.66% <ø> (ø)	⬆️
include/libtorrent/aux_/session_impl.hpp	72.04% <ø> (ø)	⬆️
src/enum_net.cpp	65.97% <ø> (+5.51%)	⬆️
test/test_transfer.cpp	76.71% <100%> (+0.1%)	⬆️
test/test_session.cpp	93.58% <100%> (-0.33%)	⬇️
src/session_impl.cpp	55% <34.61%> (-0.82%)	⬇️
test/test_upnp.cpp	71.32% <57.14%> (-4.57%)	⬇️
src/upnp.cpp	46.03% <59.21%> (-0.53%)	⬇️
... and 42 more

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