lower TCP keepalive timeout for SOCKS5 UDP tunnel

[arvidn]

to detect network failures and retry

[codecov-io]

Codecov Report

Merging #4290 into RC_1_2 will decrease coverage by 0.04%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           RC_1_2    #4290      +/-   ##
==========================================
- Coverage    74.2%   74.15%   -0.05%     
==========================================
  Files         438      439       +1     
  Lines       60719    60739      +20     
  Branches     8975     8979       +4     
==========================================
- Hits        45056    45043      -13     
- Misses      11171    11216      +45     
+ Partials     4492     4480      -12

Impacted Files	Coverage Δ
include/libtorrent/aux_/keepalive.hpp	100% <100%> (ø)
src/udp_socket.cpp	50.13% <18.18%> (-1.08%)	⬇️
src/timestamp_history.cpp	62.96% <0%> (-29.63%)	⬇️
src/disk_io_thread_pool.cpp	65.85% <0%> (-6.1%)	⬇️
src/ut_pex.cpp	53.54% <0%> (-5.32%)	⬇️
test/swarm_suite.cpp	79.09% <0%> (-2.73%)	⬇️
src/pe_crypto.cpp	68.44% <0%> (-2.14%)	⬇️
test/web_seed_suite.cpp	89% <0%> (-1.5%)	⬇️
test/test_ed25519.cpp	97.26% <0%> (-1.37%)	⬇️
include/libtorrent/torrent.hpp	80.24% <0%> (-1.24%)	⬇️
... and 17 more

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[arvidn]

I was hoping someone would test this patch. But I feel pretty confident it won't make things worse