Random username if none given

[cameronangliss]

Requires #704

We change the default behavior of Player's username generation to make a random lowercase alphanumeric string of length 10. This makes it so that if one is trying to, say, pickle their environment in separate Pipes (which supersuit does for parallel environment running stuff) you don't have username conflicts of the parallel environments due to the fact that each environment doesn't have access to the same CONFIGURATION_FROM_PLAYER_COUNTER object.

With 10 characters, the likelihood of 2 random players having an overlapping name is 1/(36^10) = 0.000000000000027351112%

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 84.00%. Comparing base (f458350) to head (eea96ce).
Report is 130 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #701      +/-   ##
==========================================
+ Coverage   83.38%   84.00%   +0.61%     
==========================================
  Files          39       42       +3     
  Lines        3918     4444     +526     
==========================================
+ Hits         3267     3733     +466     
- Misses        651      711      +60