[service] Add a reusable CompilationService runtime.

[ChrisCummins]

This adds a new CompilerGymService class in both C++ and Python that
takes a concrete CompilationSession subclass and provides all of the
runtime logic to start and manage and RPC server that responds to
requests and dispatches to CompilationSession instances.

Issue #254.

[codecov-commenter]

Codecov Report

Merging #270 (13c5f37) into development (8aacc9c) will decrease coverage by 1.88%.
The diff coverage is 30.82%.

@@               Coverage Diff               @@
##           development     #270      +/-   ##
===============================================
- Coverage        83.66%   81.77%   -1.89%     
===============================================
  Files               78       81       +3     
  Lines             4426     4571     +145     
===============================================
+ Hits              3703     3738      +35     
- Misses             723      833     +110     

Impacted Files	Coverage Δ
...mpiler_gym/service/runtime/compiler_gym_service.py	22.00% <22.00%> (ø)
...ice/runtime/create_and_run_compiler_gym_service.py	46.34% <46.34%> (ø)
compiler_gym/service/connection.py	73.99% <66.66%> (-2.77%)	⬇️
compiler_gym/service/runtime/__init__.py	100.00% <100.00%> (ø)
compiler_gym/envs/compiler_env.py	86.33% <0.00%> (-0.59%)	⬇️

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[ChrisCummins]

The reason there are no tests for the new code is because the next PR will port the existing compiler gym services to this new runtime, allowing the existing tests to cover this.