Reduce complexity of several DHT tests

[justheuristic]

• test averaging previously ran with 3 different part sizes in all 8 scenarios, now it uses default part size and only changes it in one (most complex) scenario
• test_dht_node now has reduced number of peers, but also reduced bucket size
• test_dht_node runs more find attempts for stability

[codecov]

Codecov Report

Merging #334 (7052c5c) into master (0d67284) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
- Coverage   82.22%   82.17%   -0.06%     
==========================================
  Files          66       66              
  Lines        5987     5987              
==========================================
- Hits         4923     4920       -3     
- Misses       1064     1067       +3     

Impacted Files	Coverage Δ
hivemind/averaging/key_manager.py	95.45% <0.00%> (-2.28%)	⬇️
hivemind/averaging/matchmaking.py	76.75% <0.00%> (-0.62%)	⬇️
hivemind/utils/timed_storage.py	96.93% <0.00%> (+1.02%)	⬆️