Tidy up some minor implementation details in timeout handling

[ChrisCummins]

• Timeout is not optional, it must be a float (with default value)
• Comments for CompilerEnv should not mention gRPC (this is an implementation detail)

[codecov-commenter]

Codecov Report

Merging #725 (342ba73) into development (3a5e439) will decrease coverage by 0.06%. The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development     #725      +/-   ##
===============================================
- Coverage        88.67%   88.60%   -0.07%     
===============================================
  Files              131      131              
  Lines             7936     7924      -12     
===============================================
- Hits              7037     7021      -16     
- Misses             899      903       +4     

Impacted Files	Coverage Δ
compiler_gym/envs/compiler_env.py	75.44% <ø> (ø)
compiler_gym/envs/gcc/gcc_env.py	100.00% <ø> (ø)
...ompiler_gym/service/client_service_compiler_env.py	90.85% <100.00%> (+0.37%)	:arrow_up:
...loop_tool/service/loop_tool_compilation_session.py	87.83% <0.00%> (-2.03%)	:arrow_down:
compiler_gym/envs/gcc/service/gcc_service.py	95.97% <0.00%> (-1.10%)	:arrow_down: