sr threaded mode: fix timeout

[gpotter2]

Context: threaded mode in sr(p)(1) is useful when sending a ton of packets (for instance, an arping on a big range).

• when threaded mode is used in sr(p)(1), timeout is rendered ineffective. Fix that
• use threaded mode in arping by default

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 81.56%. Comparing base (29b5413) to head (fb9975a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4387      +/-   ##
==========================================
- Coverage   81.56%   81.56%   -0.01%     
==========================================
  Files         352      352              
  Lines       83601    83620      +19     
==========================================
+ Hits        68193    68205      +12     
- Misses      15408    15415       +7     

Files	Coverage Δ
scapy/layers/l2.py	84.41% <100.00%> (ø)
scapy/sendrecv.py	87.21% <77.77%> (-0.23%)	⬇️

... and 3 files with indirect coverage changes