chore: Refactor BlockingCounter

[dranikpg]

• Splits BlockingCounter/Impl into BlockingCounter and EmbeddedBlockingCounter, so the latter can be re-used
• Adds DCHECKs
• Adds clear comments to headers

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 56.62651% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 76.97%. Comparing base (d30de86) to head (9bc288e). Report is 14 commits behind head on master.

Files	Patch %	Lines
base/malloc_test.cc	0.00%	14 Missing :warning:
util/fibers/synchronization.cc	55.55%	12 Missing :warning:
util/fibers/fibers.cc	0.00%	5 Missing :warning:
util/fibers/submit_entry.h	85.71%	2 Missing :warning:
util/proactor_pool.h	66.66%	2 Missing :warning:
util/fibers/fiber2.h	80.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #225 +/- ## ========================================== - Coverage 77.60% 76.97% -0.63% ========================================== Files 103 104 +1 Lines 7824 7771 -53 ========================================== - Hits 6072 5982 -90 - Misses 1752 1789 +37 ```

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[romange]

Please rebase @dranikpg